So, I figure it's Saturday, and I should probably make an update as to my progress thus far.
I've got all of FreeBSD's libc/locale subsystem compiling outside of the fBSD tree with the exception of localeconv.c, nl_langinfo.c & setlocale.c which I will finish for tonight or tomorrow evening most likely.
That was all mostly just a bunch of boring monkey work, but it'll make work later a lot easier. I've also been pondering about the best way to link together these objects as something I can encapsulate within a larger library that'll export the symbols I actually need (my linker-foo is weak).
I also need to inform myself about a way to convert the locale on-disk format that Solaris uses in to something that these fBSD functions expect. Since I'm almost certainly not allowed to break things, I'm thinking it'll have to be a runtime conversion with some wrapper functions. Given, I haven't looked too far in to things, so I may be completely off-base with what I need to do here.