I think glusterfs has a shim library that will intercept some calls to glibc and interpose its own functions. So you might call write() and end up getting a userspace version. So I guess it's not outside the realm of possibility. Of course, glusterfs could theoretically intercept msync as well-- I don't know if their shim library does or not.
I would guess that the people using msync to check whether an address is valid are using it more because it doesn't require you to have any open file descriptors, than because they're being "careful." In fact, it's not even clear according to the man page that you can use msync on memory that wasn't allocated with mmap. I really have no idea what msync is "supposed" to do on memory allocated with brk, for example. So it's just another case of people relying on some pretty hairy implementation details.
As far as I can see, your best bet for checking address validity probably is "mincore." It definitely doesn't make any sense for a shim library to intercept that function.