Milestag really sounds promising.
I will get on the topic when my friend is free to work on the circuits.
I contacted http://gprsystem.co.kr/
about vehicle hit dedection, talked about weak points etc, they seemed to like the idea.
I offered them help on the topic AND my friend (who is doing research & development) is interested in GPR topic, so really neat stuff can come out with some luck and attention from the GPR systems company.
Basicly it can be GPR system for vehicles, apart from shooting vehicle gas tanks and engines until they go pop, there is a chance to use c4 packs and stuff like that more realisticly on the vehicles.
I understand honor system is the best for airsoft games (especially long ones) but vehicles don't feel pain, so they are really tricky to import into the games.