Can they be manipulated like GM tones with velocity and aftertouch?
Yes. Velocity is very well implemented in the .sf2 protocol. Aftertouch is tricky, but I figured it out and incorporated it in Ice Kold Tekno orkestra. You can also use filters and resonance that GM does not have in its specification. IKTO exploits every possible parameter in the sf2 format, including resonant sweeps, velocity routed to resonance and filter cutoff and more.