AFK wasn't a mod. It was a part of a server plugin (AdminCmd) built specifically for Bukkit using Bukkit's API.
There is a Forge mod (ForgeEssentials) that sort of replicates that functionality, but when I tried it locally before launching the current server, it was a big mess with other crap glommed in and would take a fair bit of tweaking to make not suck. There's also a Forge mod (BukkitForge) that purports to reimplement the Bukkit API, and in theory Bukkit plugins like the one we had would "just work", but I haven't even given it a try.
I'll add this to my list of things to try this weekend!