I'm by no means an expert, but what I do know is the slide pack weighs above the 130kg mark, as with all aircraft with outward opening doors, it would need to be at least as far open as the width of the slide pack on both sides to allow it to drop out. The way it was explained to me is, as you say with the 737, the pack must drop out sufficiently to pull the pin on the charge which inflates it.... my guess would be at least the 15cm as on the 737 and probably more for the 747 as the door is larger...
My particular manual doesn't specify a distance, but I have heard of the doors being inadvertently opened armed, and the pack not inflating as the slide pack was stopped before reaching that point. Not sure how your company could rectify this aside from having the truck platform at a height which would 'catch' the pack should it deploy...
Do you deal with just the one airline or many? Might be something your company needs to ask its clients for more information about, as well as looking at the procedure for who opens the door and when. 737 not so bad as you guys can actually see if the slide is engaged or not (if the door is cracked) Slightly more difficult with the types using arm/disarm levers, unless of course they are the type which automatically disarm when opened externally.
Speaking of emergency slides, this is slightly OT but I find the design on the A380 arming lever flawed (in my opinion) in that it shows green on the ends for both ARMED and DISARMED... surely a mistake waiting to happen for a tired CC??? Or are they trying to say green for in correct position for either mode, red for not fully armed/disarmed? I find it confusing to say the least!
PS- Im just a humble cc but bow to the knowledge of the experts here, so anything I've said wrong please inform me!!! :P