We were able to get out of several of the Mission maps using the "Rocket Jumping" technique. There appears to be an almost infinite (well maybe not infinite, but definitely vast) number of things you can do throughout the various levels, and we decided to capture a few here...
Video I: Large Size Video: http://www.megaupload.com/?d=UF31YDNO ( ~62 Mb), 640x480 larger size picture
Smaller Size Video: http://media.putfile.com/capaignglitchingSMALL
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Video II: Getting under Mission 13... Large Size Video: http://files.filefront.com/mission13LARGEwmv/;4936603
( ~37 Mb), 640x480 larger size picture.
Smaller Size Video: http://media.putfile.com/outofmission13 
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Video III: Getting under Mission 1... Video: http://media.putfile.com/outofmission-1
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~~ NEW ~~ Video IV: Invisible on Mission 4... Video: http://media.putfile.com/pdzmansion1 
* It appears only the host (starter of the game) can do this. When you do this, you can't move around, only look up/down. It only makes you invisible on your own screen.
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Video V: Getting out of Mission 6... Video: http://media.putfile.com/pdzriver2 Video VI: Getting out of Mission 6 another way... Video: http://media.putfile.com/pdzriver1 
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If you use the same "Rocket Jumping" technique in Mission mode and make a video, then please post it in here to keep all this similar stuff easy to find and view. Thanks.