I can't seem to find something like that in phobos.
Is it missing or am I overlooking it?

I don't think there are any, searching for GetProcAddress only returns loader.d which seems to be an old module for loading executables but probably unrelated to DLLs (?).

There's dll_helper.d in druntime\src\core\ but it only has some TLS workaround features.

I've found some good solutions for loading DLLs in the Derelict and DSFML projects though.