How about a length of resistance wire mounted on the body and a wiper attached to the slide? Same effect but no spring to work against.
The problem I see with the string potentiometer is the return spring. You have to work against it and I think it will limit return speed.
I'd been looking at this problem myself. I see that very thin nichrome wire is ~40ohms/foot and videotape is about 100Kohms/inch. Any suggestions for something in-between?