Greetings,
This seems to be a bug rather than a feature request, but maybe it slurs the boundary.
Upon importing a MusicXML file, Reaper disregards some notehead information. Two screenshots will be posted showing a few things in a moment. This was realized after noticing another user mention on the forum that proper "percussion" notation is still not implemented in Reaper. Since percussion makes use of different note heads more often than many other instruments, I attempted to try a few things and noticed a few problems.
First, the guy talking about no percussion notation is right: Reaper doesn't have an actual "Drum mapping" ability for notation. There's no means to equate for example D6 to be something specific like a G on a percussion staff. The percussion staves of Reaper follow whatever the actual midi information is chromatically.
This post is more for the fact that not all notehead-types are registered from MusicXML import/export within Reaper. To test this:
1) Open up a notation view of a midi clip and change some noteheads to be a bunch of differences. Here's the result for an example going down the notehead types list in Reaper:
2) Export this to MusicXML format....
3) Delete the original item, then Re-insert that very MusicXML file that was just exported into Reaper
The results:
Notice that some of the noteheads were retained, while others were not. Some were even switched around to something else. This most definitely needs some fixing, even if it's not very important comparatively speaking. On the journey of getting (hopefully it's planned to do so) percussion notation set up with custom mapping abilities in the future, this will also need to be considered.
Hopefully this helps, and "Good Luck".