Short term any light oil (WD-40, etc.) will keep them from rusting and is easily removed when you decide on a permanent coating.
As for a permanent coating, I'm sure that you know that STEN mags are terrible to load more than 25 rounds and even that is tough. Also, they don't always feed the best either. That said I would look for something that will be very, very smooth on the inside. Bluing or some other dunking type coating is probably the best because unless you can do electrostatic spraying getting in the long mag body with a thin and even coat will be a PITA. Parkerizing is easy, doesn't require much equipment* and is a dunk type coating but typically rough. If you can do it quickly and get good coverage without adding to the roughness and then hit it with a good teflon spray you might be OK. I've had pretty decent luck doing that. With parkerizing it's not too bad to paint the outside if you so choose.
How many are you talking about? How bad are they on the inside? STENS are really not worth putting a lot of money into but if yours are successfully modified then they are worth something.
* I have done parkerizing a few times with just a cheap stainless steel food service pan, like you see at buffets, and using my BBQ grille as a source of heat. Always came out well but it's a fine line between under coated and over coated (converted really). It seems to depend on the exact formulation of the steel so you have to watch each piece separately. I did some STEN mags of various producers and noticed that.