BIP9: What happens if I make a segwit transaction after lock in but before activation?
The new rules are come into effect only when the softfork activates.

I am not completely sure, but I think in the case of segwit transactions these would be treated as non-standard nodes and thus not accept it in their mempool. If a miner does have it in their mempool, they will not mine it into a block.

Hopefully, everyone that receives it would immediately drop it, but I think that third parties could pick the input up with an ANYBODY_CAN_SPEND transaction. They'd need to deposit the transaction directly to a miner that mines this type of non-standard transactions though for it to be mined.