I am not sure if I am understanding, if I have just enough time before I know that I will be attacked to remove say the two PPC's off of my warhammer and not having any spares on hand replace them with LL but as I plan to replace them when I get the parts so I do not add anything to make up the weight how does this abuse the system? I think it is inline with the tabletop but not 100% sure, I know you can be under weight.
I just tried this on an old save:
1>Select whatever mech that you want to take components off of and change it to salvage.
2>Remove said components.
3>Change the mech back to repair.
The mech should be active at this point (unless you took out something that it can't operate without)
4>When you have the parts you need, or have the admin try to get them for you, go to customize unit and either select a refit or make one.
I tried it with a Flashman by pulling off one of the large lasers. The mech was already missing the AMS and ammo bin (due to damage). Once I changed it back to repair, it was available for use. In the same save, a couple of mechs are missing actuators that I don't have in stock. As long as the mech is functioning, MM will let you use it.