I know the answer is probably not, but I was wondering it there was some cheat to assign a different phase function per particle system? Like rendering in the same pass a PRT loader that is using Phong shading and another that uses Henyey. Kind of like you would do with materials. Assign a different shader per object. I could definitely render the two in different passes, but then i would need get into some kind of deep compositing mess to try and match how they intersect.
Cheers