Working with additional instrument xml files?

For convenience I'm trying to set up a way to work with a variety of instrument definition files that go with different soundfonts, without having to re-edit the preferences every time -- and preferably without too much cutting and pasting of files I get from other sources.

For example, in one score I want to work with the Aegean Symphonic Orchestra definitions, in another the MuseScore defaults plus a few custom instruments with added channels, etc.

I see that you can specify both a default instruments file and one additional one. Is there a way to specify more files, or structure one file so that it includes others? I grasp the basics of xml but am not expert -- I was able to find two theoretical ways to include one xml file in another (external entity references and Xinclude) but was not able to successfully use either in a way MuseScore recognized.

It would actually be nice if you could associate a set of instrument definition files with a score, the way you can with soundfonts. I'm not aware you can do that, though. Thanks for any help!