If chains forked, can the same asics mine on either chain?
That depends on why the chain forked.
If so, could a majority miner attack the weaker chain?
Sure. But to do so, they'd have to abandon mining on the chain that they aren't attacking. Since they were a "majority miner", that would leave their preferred chain much weaker and open to attack itself.
Could the weaker chain modify their own protocol to brick the asics?
Brick? You mean damage them? Probably not. But they could adopt a protocol that the current bitcoin ASICs would be useless for. Those ASICs could still be used on any other coins or forks that used bitcoin compatible mining.