Originally I was looking at using a pot just like you say -- but the least expensive ones I was finding in Digi-Key were well over $5.00, with some over $10. The one I'm looking at now is $1.50 in quantities of 1, and I can mount it on the PCB which will eliminate some hand wiring (which is always expensive).
I like your idea of an adjustable friction source -- I want to keep the friction as low as possible initially, because one should know how to wring the most performance from a well-behaved plant, but being able to increase it would be good, too.