Add capability to center text in measure.

Well, there are tons of uses, really probably most text you'd add to a measure should be left aligned regardless of whether there happens to be a centered rest in the measure or not (eg, tempo, rehearsal mark, etc). But a way to center text would certainly be nice, and potentially useful whether the measure happens to contain a measure rest or not.

I just needed to add "G.P." to a whole measure rest and was rather surprised that it didn't center horizontally. The text is supposed to be attached to the whole note rest which is centered, so this just seems to be a bug?
Having said that, I accept this is a bit of a special case and for left-aligned text attached to a whole-measure rest I'd want it left-aligned within the measure.
The internal engine does have the ability to align text like this (e.g. D.S. markings are right-aligned within the measure), but there's no way to access this functionality for regular text.

However I'm wondering if for text attached to whole measure rests it shouldn't just do this by default (essentially, treat the whole measure as something taking up the whole measure, which effectively it is).

Or should/could a G.P. be done using some other sort of symbol - like a custom articulation/fermata?

Definitely not a bug, as mentioned - the vast majority of text is supposed to be left-aligned even in empty measures - tempo markings, instrument changes, chord symbols, really virtually everything else. "G.P." is actually a quite unusual exception. But worth supporting better someday for sure, hence the Suggestion.

BTW, a possible workaround for that special case - try adding as Lyric. That's the only kind of text I can think of that attaches to the rest directly instead of to the segment. Fingering would be another but it isn't supported for rests. Anyhow, anything attached to the rest itself as opposed to the segment will be positioned where the rest is, so in that sense, it's actually a better solution than automatically centering the GP. Probably there should be another text type just for that.

But what's the use case for text centered on the start of an empty measure? Or right aligned with the start? The lyrics suggestion is a good one, will try later.

If we take the "Fine" marker from the "Repeats & Jumps" palette and apply the Center option, we can get a flexible Centered Text.
We can replace the "Fine" text here with the text we want.
It may be necessary to set the Marker type and Label as well.

Yeah that works too when it's a full measure (which it is 95% of the time), and the default font size for a "marker" is probably closer to what's wanted.
However it doesn't solve the problem of when you want a G.P. centered over a regular (non-whole-measure) rest. I've found such an example in any scores I have but in principle "G.P." as an instruction is useful to let players know that the entire ensemble won't be playing for the duration of the rest, which may well be quite long if it's a slow tempo and, say, a dotted 1/2 note rest.
Lyrics work nicely for that though, just a matter of deciding on a font size etc.

Text centered at the start of a measure would be a common alignment for rehearsal marks, also chord symbols, annotations for education materials, etc. Whether the measure is empty or not is immaterial - these markings would almost always be aligned with the start of a measure. Left aligned, right-aligned, or centered according to the editors preferences and available space, but again, virtually always aligned with the start of the meaasure.

Good point, though rehearsal marks are frequently centered over barlines too. I'd agree both options are useful, though it's hard to be sure which is more likely to be expected in various circumstances. It would actually be very simple to implement as a flag - whether the text alignment was based on the whole measure or the segment time position (just not sure what to call it!).

Right, the norm in classical is center for rehearsal marks. But left-aligned is quite common for textual markings like "Verse" etc, as is right-alignment (the latter especially in any style involving chord symbols above the staff. The point being, we can't take away the ability to set different alginments with respect to start of measure. We simply need additional settings to also align to center or right of measure.

And, for that matter, additional settings to control wheat that alignment point actually is, particularly in the start-of-system case. Especially in the case of measures starting a system, should it be at the left edge, at the clef, at the key signature, at the first note, etc? We went through a pretty extensive round of refinements a few years ago looking at various references as well as published scores across a variety of genres to come up with specific default alignment points for left, center, and right. These cover the majority of cases decently well, But there's always cases of someone wanting some additional possibilities not covered adequately by these basic three. Any new "align to measure" facility should probably include the ability to control those sorts of things.

https://musescore.org/en/node/327802 shows another possible use case for this feature.

+1 on this. In my case, it's for numbering repeated measures on a drum part. I tried adding a "Fine" then changing to centered and the text to the number, but this didn't work because "Fine" shows up as system text, not staff text. These numbers apply only to the drums staff, not all staffs.

Numbering of repeated measures is already supported directly, just enable it in Format / Style / Measure Repeats. But indeed, centered text for general purposes will be nice someday too.

That setting seems to apply to the whole score. I don't want the repeated measures numbered everywhere--just for certain measures in the drum part. Is that possible?

Not currently. But why are you thinking others wouldn’t also appreciate seeing these counts? As a pianist I certainly do!

In my case, the parts already exist and are printed. I am transcribing the parts to create a full score for the conductor, who needs to see the parts in the score reflect exactly what the printed pages show (even the mistakes, and there have been a few of those).

+1
Would be very useful in creating drum charts with alternate time options for repetitive sections that are generally improvised.

E.g.

As this issue tracker has been discontinued, request it again on GitHub