Spanners, which A/C? Apologies, did my apprenticeship on the 747 in heavy maint. Can't imagine trying to change a L1 or R1 window at the gate
However you're right I've heard stories of HAECO changing engines, landing gear at the gate so never say never.
What I should have said is it'll take a lot longer to get the required SOE on the line, I found C checks were usually the best for collecting SOE.
I came across a line LAME having had the main deck freight moved was getting the floorboards lifted on 747 freighter to get access to a cargo fire detector / potable water pressurization problem, they could hear the air leak. He was surprised when I said why aren't you just going through the forward cargo, when I showed him he said sorry he's never done heavy maint and didn't know that access was possible through fwd cargo.
Never got the opportunity to do line maint, but would imagine a solid heavy maint background would be of benefit, however I know some heavy guys that aren't comfortable with the speed of line maint or the use of the MEL and would rather fix it to the detriment of OTP. But that would come with time on the line.