As far as I can see, the answer is no, the flash cannot open if anything, is plugged on the hot shoe of a Canon EOS 450D:
Can anyone confirm that I wouldn't be able to use something like the Jobo photoGPS and the flash at the same time?
Yes, you could use a sync cord (or an optical slave) with a hot shoe adapter to provide another hot shoe for your GPS unit. Since the GPS unit doesn't have camera brand specific models, we can deduct it does not need TTL info, just the basic flash signal. Based on this thread it seems using a sync cord will not disable popup flash on your camera, but you might want to test in shop to be sure.
There is a sensor in the hotshoe to detect a flashgun; this inhibits the pop-up flash from coming up (or even firing, even if you trick it). You might be able to chip off the relevant part of the slide so the camera doesn't detect the hotshoe flash, but I wouldn't personally recommend it.