Hi xfrocks. A very good gesture I must say. The way to do it, is very simple. Just go to the Addon's thread which is abandoned and post the updated Addon as an attachment in a new reply in that very thread. That way you haven't created a new thread and no one can say you are "taking over" the addon and the community can get the update. I did this with a couple of addons I used in vb.org which were abandoned. Basically what I did was since I was using that addon and updated it for my site, I posted my updated addon in the Addon thread. So other people if they wanted it, they could take a look. No obligation of support on my part and people get a new update and the addon author if he returns, he can do as he pleases!
Of course this solution is only viable if the addon thread is still open. (Hence my objection a few months earlier when one particular Addon author was posting Addons and asking his threads to be closed.)