I would like to run PackRotamersMover to pack on a rotamer set which I specify. It looks like the setup code calls core::pack::pack_rotamers_setup() which builds the Dunbrack rotamer library. I'm guessing such functionality is not supported, maybe there is another mover which provides it?
I don't think there's any simple facility for specifying your own RotamerSet for use in packing.
If you're working on the C++ code level, you can approximate pack_rotamers_setup() yourself by replicating the function calls in the implementation from src/core/pack/pack_rotamers.cc:189. Instead of using the RotamerSets setup there, you can supply your own, constructed by whatever method you wish.
I am at the C++ level and this was my fallback option. Looking over the code it seems pretty straightforward to extend this functionality; I'll post some code when I have it working in case anybody else needs this.