Don't know. I guess I'll fool around with it some. I did download the source and found a function named something like switch_usb_mode that is compiled differently if PTP is enabled, so maybe that is what I'm looking for (but I don't know if the standard build for the A480 enables PTP either). I want to be able to use the USB remote to trigger taking a photograph, but then I want it to enable USB downloads for a bit after that so my computer can grab the photo, so it gets tricky using the USB line for two different things (but in my experiments, I did get it to work with SDM).
It's currently set_record and switch_mode_usb in CHDK, I think. The former doesn't work on many cameras if PTP is active, so you need to use the other one to return from play to rec.
So I think your (Lua, but ubasic probably has the same commands, just lose the parentheses and comments) script should have something like
...
set_record(0) -- switch to play from rec
sleep(60*1000) -- give the PC a minute to connect and transfer images
switch_mode_usb(1) -- switch to rec
...
but I really haven't tried this lately and even if I did, it could be different for your camera since mine is a vxworks camera.