I've been working on a Rosegarden studio device file for the Roland SC-8850, and I'm finally done (unless I notice any inconsistencies or typos I haven't already corrected). I just submitted it to rosegarden's devel mailing list for consideration, but since it may take a while to end up in rosegarden's library, I chose to post it here too so others can try it if they use Rosegarden.
The most useful for music creation is the studio device file with the separate CM-64 map, the rest of the maps being merged into one SC-8850 device. Both devices can be applied to the same midi port/device anyway. That way, the different MSB/LSB method for the CM-64 variations is easier to navigate in Rosegarden's GUI.
The "single-device" file is there just because I tried it (merging all maps), but due to the CM-64 map's MSB/LSB difference, it's awkward to browse when creating music (certain CM-64 variations are grouped under capital tones they aren't related to).
The easiest to edit is the one including "all-separate-devices". Also, the compatible maps may be imported into other similar devices' studio files (although I'm not sure Roland kept the variation names exactly the same for each incarnation in this line of sound modules).
I believe this is mostly compatible with the SC-8820 too, but the documentation seems to say there should be a difference in the number of tones between the 8820 and 8850. The difference isn't in the 8820 studio file, though, which only includes the 8820 map. (I used it as a base for the 8850 map, obsessively editing wherever I noticed differences in names, but I never had to add, remove, or change variation numbering).
I Noticed & corrected CM-64 LA map's "Flue 1" to "Flute 1" next.
I finally added a few wanting features. They now include an additional duplicate SC-8850 map for the bank selected by the button on the front panel (only the SC-8850 map, but at least it's a superset of all but the CM-64 variations) allowing for greater GS sound module compatibility, all controllers defined (except aftertouch, which doesn't seem possible at the moment) and all drum set keymappings! I hope That's useful.
Edit 2017-11-22: And this should be the final version until I notice the inevitable typos. This time I only corrected a few structural issues aside from the metronome device and related device IDs. Thanks for your patience:
Now that this is available to me, I'll be slightly more likely to create midi compositions, but still, I never learned how to play piano.