Closed and Open Hi hat notation as per Trinity College London

Both are available in the Articulations palette (of the Advanced workspace)

As Jojo helpfully pointed out, the + and ○ symbols can be added manually to get that kind of notation - which in my experience is the standard and what drummers would expect, at least the part about open hi-hat. MuseScore's default to place it on another line feels reasonable given its current limitations, but I've never seen it elsewhere, and would be curious to see how many drummers would understand it intuitively.
(I have toyed with using another notehead like XCircle or With X for open, but that has the same issue of being non-standard, and also looks messy and less readable.)

Currently, my workflow is to write all (unique) parts that involve hi-hat using the default settings first. (Copying&pasting measures later is fine, but entering or changing notes is annoying.) Then, to get the proper notation:
1. Right-click on the staff to Edit Drumset and place Open Hi-Hat on staff line -1 (same as Closed).
2. Right-click on an open hi-hat notehead, then Select → More... and enable Same pitch & Same staff to select all at once, then add the circle symbol ○ ("Open") from the palette. (Or "Harmonic" for a smaller circle.)
3. If the + symbol ("Muted (closed)") should appear over every single closed hi-hat (or most of them) as well, repeat step 3 for them.
4. Where needed, add/remove symbols manually - perhaps + only once after ○, or when staying open/closed for repeated notes or whole sections... (I'd love to learn more about what the most common practices are here!)

The symbols are placed way too high by default. That can be fixed by setting their Anchor to Above Chord in the Inspector:

We could just make that the style default and be done with it, but that will affect ornaments (trills, turns, mordents...) on all other staffs as well - which is probably not desirable, outside of a drums-only score/part? (I'm not sure I like so much whitespace for ornaments either, but I'm not as familiar with ornament-dense scores like e.g. for violin.) So to be safe, I usually do:
5. Select every ○ using Select→More with Same subtype (& Same staff), and set Anchor to Above Staff.
6. Repeat step 5 for +.

(Confusingly, + is classed as an ornament in the Inspector - whereas ○ ("Open") is classed as an articulation, and so is the "Harmonic" circle. All 3 are grouped into the Articulations palette, but they seem to share the style default setting with ornaments...? 🤔 I'll create a bug report after posting this.
EDIT: #319293: Inconsistent labeling & style behavior of articulations (esp. "Muted (closed)"))

Ultimately, I can make standard, readable, pretty drum scores in Mscore - and that's great!
It sure would be nice to get this by default, though.
Putting the above steps (except maybe 4) into a plugin seems straightforward, and I'll probably give that a shot when I find the time. However, it's still a destructive, non-procedural workflow:
Let's say I don't want to wait to see it in standard notation before I'm finished writing, or want to go back and make changes to my score later. Then I'll have to deal with notes that look identical but sound different while writing, until I add the symbols manually or run the plugin again. I've learned to deal with it when I need to, but it's still bad UX. (Which is also why I do step 1 each time, instead of using a different default drumset. (Is that is even possible from instruments.xml?) Actually, the solo Hi-hat instrument has this problem by default.)
Finally, if it doesn't happen completely automatically, users not too familiar with drum notation probably won't even notice it, until maybe some day their confused drummer complains.

Long story short: My (ambitious) feature request would be the option to tie an additional symbol or element to a pitch in a drumset (under Edit Drumset, alongside notehead group, stem direction...). The symbol would be added automatically whenever this note is entered, but could be selected/tweaked/removed separately. It should also be added (or removed) whenever a pitch is moved using Up/Down-Arrow keys.
(Changing the pitch when a symbol is added (e.g. turning a closed into an open hi-hat) would sure be cool as well, but feels like less payoff for even more dev work & convoluted special-case code/UI?)

Besides hi-hat, other use cases for symbols tied to a MIDI pitch would be:
- crash cymbal with l.v. (open tie) - not ubiquitous, but can improve readability
- choked cymbals (closing bracket or comma)
- rolls (tremolo lines) - using a dedicated sample could vastly improve playback
- distinguishing which part of a drum or cymbal to strike (bell, rim, center, slightly off-center, ...)
- dozens of custom extended techniques in modern art music, I assume - distinguishing many instruments/techniques within the same percussion staff by notehead and position alone can be less readable

The placement anchor issue (steps 5-6) might be partially resolved by not grouping +/○ with ornaments for purposes of the style default, and/or by including positioning settings in the drumset definition as well.
As for a more general feature (not limited to unpitched percussion entry) that would help here:
Separate style defaults for different kinds of staffs in the same system. (For articulation placement, but also perhaps stem shortening?) Vocals would be another case where this would be very useful.