I've already written a few personal xbmc scripts for my own use, so I can use that knowledge =)
I already thought up how to do it, just need to find time for it since I work during the day (as a software engineer) and many things consume my non-working free time.
I wrote a FTP client with python before (not for xbmc, but for your PC), so I can literally use that with some modifications for the FTPing between xbins server and your xbox. Zip/RAR isn't a problem really, there's python libraries that handle those sort of things. I absolutely hate designing interfaces, but I can thank Textbook's pic on the thread for making that part simple

His idea for the interface looks clean and simple, which is good. The IRC thing isn't that big of an issue, if you think about it, you don't need a full IRC client (likewise, you would not need to support FTP uploading, only downloading is fine). All you need is to connect to the server, msg the #xbins channel and wait for the proper response. I would forgo looking at any existing IRC source code (because we aren't looking for a IRC client, we just need it to do exactly what we want, aka msg xbins !list to get proper auth to login to the FTP server). All that's necessary is to create a simple socket connection and send the right msgs.
http://tools.ietf.org/html/1459 is the official IRC protocol documentation won't be too hard to piece together the exact packets you want.
My biggest worry (outside of my own time constraints), is the fact that whether or not the xbins people would be happy or upset that I created such a thing. I'm thinking they made people go onto their IRC channel for a purpose before downloading off them, and this would seem like a "cheap" bypass of that system. I wouldn't want to start this project until I can get a favorable response from them, since they've done so much for the xbox homebrew community, I wouldn't necessary want to cause them grief.