Well, on other forums, we just use +1 (this was before Google integrated the same method) the only problem I see with that here is there is no reply tier on Gimp Chat, it's just stacked (all posts stacked on each other without any indentation). So, you'd be required to quote a user's post and then +1, which is really what you are wanting a simple button click for, right?
The point system style mods normally place clickable icons in the body of each post so you don't need to reply to give someone +1 (or -1). Just click the icon.
The "Like" mod places an icon at the top of each topic. When you click the "Like" icon, it adds +1 to the "like count" for that topic and optionally places your username on a line underneath, displaying something like this..
Members who like this thread: mahvin, PhotoComix, GnuTux ...and so on.