I've never had a problem with the battery caps on my 75's. I'd be wary of replacement caps for two reasons. (1) The threading on the cap itself may be fine and that its the threading on the mod that's worn, or that the new cap's crisp new threading might cause more problems with the worn threads on the mod. (2) The Pico has TONS of clones, and the caps for the clones are slightly different than the real ones. A friend of mine has a clone and a real Pico, and he has to be careful which cap is which.
I'd just buy a new mod, or perhaps try a single, tight layer of teflon tape on the male threads.
I'm in the market (soon) for another two battery mod, and the Invoke is on my short list. Another Battlestar, the Charon TC and the Voopoo Drag are, too.