I was unable to add a library from a build.mk in APPDIR. I think it has to do with the order things are called in the main modules make process; the problem is you need the updated LDFLAGS when the .elf file is built, and a LDFLAGS change made in the user make don’t seem to affect that.
However, when I just hacked the .elf file generation to include the CMSIS library .a file, it failed to link because it was too large to fit in flash. So that will be a problem as well.
I think there might be a reason why one of the posters in one of the other threads had to use a monolithic build.