every time I want a tank and see someone doing this I steal his tank from the repair pad if he jumps out to fasten up the "upgrade" as engi.
It depends on the situation. If theres a chance for it to be destroyed from the repair pad to the VF than its greifing. If the base is O.K. you deserve a cookie for being so smart
I had the idea to do this at one point. Unfortunately, there was also a bug that prevented re-customization unless the full cost of the vehicle was available. So I just considered it useless, and never tried it again.
Only problem with this is you could end up taking longer getting the armor than waiting for the full res, especially if you have to wait for 24 plates to be repaired by the rep pad.