"He who marches out of step hears another drum." ~ Ken Kesey
I print onto tranfer paper and heat press with a cap heat press. I did some white on navy awhile back I used Excalubur 50/50 white and transfer powder. My hat customers always bitch that the bills are not flat brims though.
Transfers are easy to print and our preferred method for foam front truckers, we've done a lot of 'em this way. You want Cold-Peel paper, go for the high quality stuff for multi color work. You can print the sheets on a clean platen, lightly and evenly tacked with the rest masked out. You'll need guides and should really have a vac table for long runs and/or multi-color. You can flash the sheets on press if you like however, I've done this to build up a "HD" looking transfer. No modification is needed to any plastisol with a good about of body (think opaques here) for cold peel. You will, however want to use adhesive powder to help the ink adhere to the polyester foam fronts of the caps. You can mix this into your ink (not my suggestion for caps, not a bad method for private labels though) or sprinkle it onto the wet ink and then "thwack" off the excess by snapping the back with your fingers before running through the dryer. You want to gel the ink enough that it will not offset and isn't going to leech out all it's plasticizer in storage. A good test is to peel off some of the ink and gently ball it up. If it unfurls itself you've gone too far. If it stays balled up pretty well, all's good. The better test is to run the transfers after cooling on the heat press but the peel test helps in getting initial temp adjusted. In any case, determine the proper temp and hold it there. Expect a 6-12 month shelf life for these. Too many more details to go into right now...but union ink has an excellent white paper on transfer printing- both hot and cold peel. Read it here: http://www.unionink.com/articles/transfer.html