I finally looked into what the guys behind the utterly filthy and NSFW comic JerkCity use as their creation method. Turns out it's an old Microsoft IRC client called Comic Chat, that takes an IRC input and produces comic strips.
Naturally, as with all things, when you take something intended to produce content for children, and then give it to adults, it becomes hillarious (see what happens when you give a teenager a copy of some cartoon making software if you doubt me).
From what I understand (and I've only used it for about 3 seconds), you can drop in additional backgrounds, characters, etc. Here's a link or two - and remember, a lot of these sites are 'a product of their time'....
Pretty out-dated, but here's one: guide to Comic Chat