I made a similar temporary "tank" for a BMX frame, that's the only full frame I've soaked so far. I also added cardboard boxes underneath the plastic sheeting to fill the spaces between the main diamond and the stays, that way I was able to use less liquid (I used about 8 gallons.) I thought about using a small electric pump to keep the bath mixed and circulating, but I haven't got around to trying it out yet.

Thanks for the wallpaper tray suggestion, that looks like a great idea!

Edit - I see that I haven't directly answered the questions... I assume that submerging the frame would get all surfaces into contact with the OA, but in my case the frame only had rust on the outside (it was chrome.) I also used compressed air to speed the drying out process.