Quote: "A simple text-based system, heavy on the servers?"
If it's database driven(which I doubt), consider thousands of people writing and thousands of entries per second/minute being entered into a database...
If it's text-based still,the data has to be held somewhere even if temporary,which will require for example a log to be created for each chat(if seperate) , if it's one 'room' for all it wouldn't be such a problem,but the last thing you said sums it up pretty much.
Oh and if it's AJAX based that would require a script to be ran each second (or a bit more) which is not ideal.
Quote: "Either way, it probably wouldn't exactly be a good idea considering the quality of users around here."
Quote: "It's not a plug & play forum, the software was written from scratch and so it'd take some effort to integrate a chat system into it."
It's not about the effort...It's a question whether it's a good idea to integrate one or not.I'm sure the administrator is more than capable to write a simple chat system and it does not take much time really(I speak from experience).