not sure I fully get the idea, but after looking at the Sweep SOP setup of your example file I think you might have easier time with Limit SOP. It has simple UV mapping options and you can control color/scaling along spline with CHOP channels (similar to geo instance)
If you want 3d tails on a particle system try going Particle SOP > SOP to CHOP > Shuffle CHOP split all samples > Trail CHOP and then replicate Limit SOPs in tube mode, each Limit SOP referencing your Trail’s t[xyz]+str(me.digits). Of course this gets heavy with increasing particle count and/or trail length.