Setting amber to true changes these defaults:
The megadriver for the classic driver set will be installed to a file name unique from Mesa main. Beyond that no attempt is made to use filenames different from main - particularly not if glvnd is explicitly disabled - so you would still need to configure your install paths appropriately so things don't collide, if you want them to not collide. But if you have that problem, consider that glvnd already solves it.
This has a quiet dependency on glapi and gbm's library API remaining stable. gbm you could fix by linking it statically, glapi is a little harder. I think the right move for glapi is to use glvnd's gldispatch more directly, but that's not public API at the moment.