The other backward compat I was thinking of was using [[] as a way to get [ to appear. But I haven't seen that used recently and I only saw it used in chatter.

But I've changed my mind and am now instead thinking of transitioning to use [[...]] for PM links.

But the first step for either of these (as well as leaving brackets untouched inside of HTML comments) is having the HTML filter code handle square brackets and I've already started that work. I've got to do the next phase of testing...