For reference, I specifically recall retep talking to Gavin about it on IRC shortly after his last post, and Gavin agreed that modifying the SIGHASH system was a bad idea. I didn't feel like pressing it, at the time, though.
That's not to say that this can't be done or that Gavin and the other devs aren't persuadable. But it would mean mobilizing support for it and presenting a strong case to those that are against it. If the system was built with feature from the start, we'd all be happy and no one would have an issue with it. It's more about convincing folks that the benefit is worth the trouble of making the change. As you said... the people who most strongly support this are the ones actually developing software that has to deal with the megabytes of supporting transactions just to verify a few bytes.
You'll have more luck pushing for this by developing a multisig wallet implementation and getting people to actually use it first - P2SH required a painful soft-fork yet years later it's still hardly ever used, making people question the value of new features. P2SH was something that everyone was supposed to be using by now because of the "obvious" security need; you're proposed reason for a soft-fork and associated system-wide risk is significantly more niche.
Like it or not the amount of money that can be lost for a single consensus bug is staggering and gets bigger every day.
FWIW Litecoin will need to undergo a soft-fork soon to finally get height-in-coinbase implemented - talk this over with them, specifically Warren Togami.