Depends of course on what emulator, but it should be possible. For instance for the UltraHLE emulator which uses Glide it should only require you to modify an arbitrary Glide wrapper to enable truform, assuming the game passes normals with the geometry. How good the results will be is another question though.
I was going to write one for the ePSXe... Don't know if it would work though, as "TruForm" requires vertex normals, and I don't know if the GPU plugins pass normals...
Would it be possible in the case of Project64 in which the core source is released , but the graphic plugin is still closed ? Should I stop asking exceptionally stupid questions ?