Originally Posted by GravelMN

Go to the firearms section of your sporting goods store and pick up a flexible shotgun cleaning rod, a large patch tip and a 12-gauge cleaning swab. Depending on frame design, you should be able to get into the main tubes fairly well to swab around generous amounts of rust remover. Do this a few times a day, plugging the holes with plastic bags to prevent rapid evaporation. If you have access to the stays through the seat tube or external holes, you can plug any other holes and position the frame so you can fill the stay. You might have to do this a couple of times in different positions to reach all areas of the stays. It's a lot more hassle and not as good as dunking, but it can be done where dunking is impractical and should do a decent job. Set the frame on a protective mat or old towel in the bathtub between swabbings.

For external rust you can use a wrap technique like you described. Just swab the area heavily with rust remover and wrap in rags soaked in remover. Overwrap the area with plastic wrap to ****** evaporation. Again you might have to do it a few times to get good results. If rust is limited to a specific area like the BB shell and chain stays, you could try soaking just that area in a smaller, shallower container.

Edit: Oh for the love of Pete , the word police won't let me use the word ****** in proper and politically correct context. Please insert "slow the rate of evaporation".