Hi, I was looking for a way of easily doing this, but the bundled tvservice binary didn't seem to support SDTV, so I went looking for how to do it with the vc_tvservice APIs. I don't really understand them, but I seem to have got something that works and I thought I would share them here: https://github.com/adammw/rpi-output-swapper
Hopefully I haven't wasted my time as something else already exists that does this simpler, but at least I know a bit more about the system.
The major part of getting this to work is knowing how to initialise the APIs so they don't throw you back error codes instead of working, which is definitely not helped by the lack of overall documentation of how to use them.
Not sure if it works with an X environment (I don't use it so I don't know), but it automatically calls fbset so if your in a console it will appear on the new screen, and I've tested it to work fine with even omxplayer still running.