But since all off-chain transactions (via HTLCs) are being summarized to one transaction (last state is being published), this is not an issue.
Right, you beat me to it; by summarizing you probably meant something like this:
Upd: actually, having thought about it, something like this would be neat: if someone sends me the correct preimage, I'll just replace my puzzle output of this amount with a simple output giving him the funds, since I know he can take them anyway, since he knows the preimage. But this would clean up the outputs and consolidate them, so that when the channel closes, it's just one clean transaction signed by me and him without any hash puzzles.
Anyway, this is really really fun. LN seems to be going places.