I don't suppose it matters much at this point, but, have you tried a UI-based IRC client (like Colloquy for Mac or X-Chat for Linux)?
I don't know about any clients on Linux, but, Colloquy (and most IRC clients for Mac, I suppose) use a WebView from WebKit to display text, so they'll have points 1, 3 and 4 covered. And point 2, if you don't need chat history.
(disclaimer: I contribute to Colloquy for both iOS and Mac.)
Colloquy is the client I use when I'm on IRC, so I appreciate the work you've done on it.
Point 2 is actually the most important point for us. Since Tom was the only one that liked irssi, the rest of us lost the backlog and that was a deal-breaker.
* It just works
* Inline image viewing
* You can customize anything via Propane[1]'s custom javascript support.
1. http://propaneapp.com/