Yes, but wouldn't it be beneficial to agree on a canonical signature with positive R and S that gets preferential treatment for mining?

A middle ground is to come up with rules for preferred transactions (ECDSA sig has the preferred sign, DER encoding, only very basic scripts) and hold any non-preferred transactions until the next block to see if a preferred spend of the same outputs shows up.  But you can't force miners to do this and it's slightly contrary to their interests since they'd have to forego the transaction fee and essentially give it to whoever finds the next block instead of taking it for themselves.  Cooking up new rules also isn't the sort of thing you want to do in a crisis like the current situation.