> libgl can be dlopened, glibc can't be. That is the problem.

What exactly prevents this for glibc? I assume you'd need a dlopen equivalent from somewhere for bootstrapping, but are there other issues (like TLS or whatnot)?

Yeah, TLS is one reason. I don't remember the details, but last time I looked into it, glibc abd the loader have some private interface that they use to load the program correctly, and there are no stability guarantees for this interface. Ideally, the loader and libdl will ship with the kernel, and glibc will have to use the public interface they expose.