For the -400 any of the door open messages are either a caution or advisory. The passenger doors (all 10 of them) are an advisory and are actually able to be opened in flight for smoke evac purposes (hate to be the poor FA who had to do it though!).
Anyway, above 80 knots all cautions are inhibited anyway so we wouldn't know about it until the inhibit is gone (400'). So basically, prior to 80 knots, a cargo door coming open would be an abort as it would trigger a master caution. After 80 knots we would find out about it airborne. Our QRH calls for setting the cabin alt to 8,000-8,500 feet and leveling off at or below that altitude. While it doesn't directly say to land it is assumed because who is gonna tool around at 8,000 ft. all day in a 747.
I know of at least one hull loss in a 747 due to a cargo door coming open in flight and that lead to a design change.
As for the MEL, I'll look our up. All of our manuals are on the aircraft and online but I couldn't download it earlier. I have a paper set but not the DDG (dispatch deviation guide), which is a fancy name for the MEL/CDL. I'll look it up in there as well but I'm sure there is a procedure in there somewhere.
Anyway, back to where we were, 80 kts is a key and aircraft type is as well. In the original posters case, as I said before, it would be a PIC decision and I don't think you could fault someone for going either way in that particular case.
Update: The procedure is basically the same as the OP used and I have used on every other type I've flown. Visually inspect that the door is closed then you can go. I don't have access to the actual MX procedure so I'm not sure if they would deactivate the sensors on that door or not but I would assume that they do. That's what it all boils down to, if it is MEL'd then the sensor should be deactivated to prevent an unnecessary caution or warning.
