Hmmm ... JV partners are getting more and more wary about sending their subscribers to offers that don't have instant sales commission potential ... you'd have to show them incredibly high conversion numbers from test results on the upsell from free to have a chance, IMHO.
Have you considered a $1 trial? ... something comparable that would at least require prospects to provide payment info and toss aside the majority of the simply curious?
Do you have a list to a/b test?
If not, would you consider running test promos via newsletter ads in popular newsletters to accomplish the same?
... or have one solid JV partner that would allow you to run test sequences with their list?