CHDK in the 1.0 release would probably be OK with hot-swapping (caveat this is off the top of my head). There could be issues with the CFG file, but I think CHDK would just save the current settings on the new card the next time you went in and out of alt.
I'm not sure the Canon OS would be OK with it. Most of the cameras share the battery compartment and SD card, and shut down when you open the battery door. I didn't look carefully, but I didn't see anything one way or the other in the G9 manual.
IIRC some users reported doing this successfully a long time ago.
The development trunk (1.1) would be more problematic, since it expects to load modules from the SD card at run time. If you had all the modules on both cards it might be OK
Overall, I wouldn't do it with any data I cared about. Cards are cheap, just configure them with whatever you need. If you need to have the same scripts on a bunch of cards, just set up a batch file or something to prepare the cards or upload the latest scripts over ptp.