I just picked one of these up and started playing with it. Yes, a 5# CO2 with reg will easily stand on the hump without interrupting the kegs, with space for bottles or whatever too. I unscrewed the 16 or so screws holding the seal and door panel on, and just screwed the seal back on. I don't feel any cool spots around the door. The built in regulator is in the way of the second keg, but is only held in place by one screw. I removed the controller housing and just bent what I assume is the temp probe wire back to sit the controller above the hump instead. The higher the number on the thermostat, the lower the temp. At 1 with one full keg, it holds around 50-51degrees. I turned the knob to 7 overnight, and ice condensed on the ceiling and walls, and the cylinder of water with thermometer was at 37. It seemed like the compressor was running full time though. There is a small channel behind the hump that funnels to a drilled drain hole with no plug.
I mocked up my ball locks and converted pin locks. The ball locks would not fit two without trimming the shelf rails on the sides. The second keg sticks out the door about an inch without trimming. The pin lock kegs will only fit one. I suppose if you have a 2.5 or 3 gallon keg, you could fit it in along with a 5gal pin lock keg. If anyone wants pics of fermenters/keg/co2 tanks mocked up, PM me and I'm happy to do so until I have it in use.
Update: I cut down a whiteboard I had lying around to 18 5/8 x 31. I used the original door panel as a template to drill 1/8" holes, and remounted it with the door seal on it. Putting it back on would be easier with two people. The groove in the top of the door holds pens too