Hi Commando
I'm no expert (having just started scripting mself) but I am guessing that like everything else in the game an emplaced satchel charge is an object.
All you would have to do is attach a script to the emplaced bomb (say - "Disarm Bomb") that checks whether or not the nearest character is an engineer and if he is then allow him to disarm it (adda an action allowing him to remove the object and add one bomb to his weapon total). You might make it work MOST of the time for engineers and SOME of the time for everyone else (99% / 50% ?) or make it only work if the disarming person had the appropriate object (disarming kit ?).
The only problem is that I don't know how you could add the action to the object BEFORE it was emplaced - until then as far as I can tell it's not an object, it's an entry on the carrying unit's weapon tally.
Maybe you could set up some sort of game logic object in the target area that checked whether or not any emplaced bombs existed, and if they did, then added the appropriate action to the relevant object (use the "nearest" command).
Unfortuanetly you've reached the end of my skill in this area