I have both MotionX GPS and MotionX Drive and although I've loaded maps on MotionX GPS I have not found a way to do it on Drive. The iPhone is limited to 250MB of maps in the cache, while the iPad is limited to 1GB. It looks like they no longer support caching of Bing Maps, even though my ap still has that listed as an option (this screen capture is from the manual for the iPad as opposed to the iPhone)
I use this ap all the time in Riyadh (with a street map) and out in the desert (with a terrain map). I will have to agree that it was a time consuming pain in the rear end to upload the maps, as internet over here is over 3G and is rather slow. That said, I do not know of any other options out there at the moment, so while not perfect by any stretch the MotionX GPS ap is better than nothing I guess.
I still think you guys are crazy for even thinking about lashing up an iPad to you handlebars!