We have a "status box" on our sidebar, and we have a "change expression" option for our avatars. Why not have a "status box" instead of the "change expression" option, so that we can write a brief caption or comment about our avatar?

Why would we get rid of the change expression option? If you set different expressions on your HA (which I do) then you just flip between them with the change expression link. And I use it to see if my HA saved right when the cache is slow.

Where would the status for our HA go? On our profile? The box for the HA in the forums is already too stretched out, it doesn't need anything else lengthening it.

Yeah, I don't think I'd want to see a blurb below every avatar on the forums. You can write a note in your signature, status, or on your profile if you do feel your HA needs an explanation of any kind.