Request 128th tremolo

I've never seen 5 tremolo lines.

I haven't see everything, I'm just starting a conversation. I'm not a professional musician of any type, so I mostly learn from experience.

It's in the Bravura font so it could possibly be added.

FYI, MuseScore doesn't use the bravura font or any other font to draw tremolos. It draws lines.
That being said, there is indeed a 5 line tremolo in SMuFL : http://w3c.github.io/smufl/gitbook/tables/tremolos.html

Yes please! I need a 128 tremolo (for the snare) today for a marching band score.

Are you sure you don't really just want an unmeasured roll? You really want to see five lines through the stem and expect players to count exactly 128 strokes? Assuming you really just want an unmeasured roll, I'd add a more generic symbol for that, then if you also want the playback to happen to use 128th notes, just add that on a separate invisible staff. At least, that's how to get the job done today. In the future it would be nice to have an option for "unmeasured roll", maybe something in the Inspector to say how many actual strokes there should be which still displaying only a single unmeasured roll symbol.

In any case, since the five-line tremolog symbol exists in SMuFL, you can add it today, so workaround exists, with playback being handled manually.

Isn't that (an drumm roll) expressed with a Z though the stem?

That Z is available in the Tremolos palette