I'd really go with querying if vehicle (list thatlist select _N) != (list thatlist select _N), so he must be in a vehicle. That will surely affect anyone in the list. So set the trigger up as usual to trigger on anyone and on activ:
[] exec "belowscript.sqs"
_thatmax = (count list thatlist)-1
_t = 0
#thatloop
? (vehicle (list thatlist select _t)) != (list thatlist select _t) : (list thatlist select _t) setDammage (getDammage (list thatlist select _t) + .05)
~.1
? (_t <= _thatmax) : _t = _t + 1; goto "thatloop"
I'm sure if that works it's the shortest way. O n dat adds to the damage they got, not sure if you want that but you know what to do, don't you?
If you have many of these triggers, you can try passing the list name into that script to avoid creating multiple script for 1 purpose.