Personally, I don't think it sounds like a good way to go. There's another PR that actually goes further to const the descriptors, so they can be entirely in flash, instead of being constructed into ram as they are now. (this is a big improvement for ram usage) But, I'm not goin to be doing any major work personally anyway, so see what other support you can drum up if you think this is a meaningful case. (You're the first person I've seen request it though, so perhaps consider if you can do this in other ways? pointing to alternate consts perhaps?)

Yeah, I've seen this PR and I'm pretty aware of the fact that vast majority of developers are not interested in changing USB descriptors in runtime. I'm happy to be the first one here :)
I was thinking about creating a macro that could be either 'const' or empty depending on some definintion in Makefile.
I know people hate macros but that sounds like a pretty easy change. Of course it may be easy at first glance but in reality it may turn out to be a train of questions like which consts to macro-ize?, should be there some groups of cosnts, etc.
Anyway, at this point I'm going to create a fork of libopencm3 and use it for my project. Maybe I will come up with some PR for that in the future.
All I wanted was to see what's your opoinion on this kind of change and now I know it's not a song of the near future :)