Well, if you like Invision and XChat, you can learn Perl language and "translate" invision into perl (or TCL).
But writing another IRC Client needs a lot of a work because of the security aspect. It's very very hard to write an IRC client without buffer overflows or something like that.
It needs a very good knwoledge in both programming and security.
I agree with you that it would be nicer to have most of invision features as native functions of an irc client but it takes time and I'm sure you wouldn't be happy if you read that due to a programming error all your datas are compromised.
And for iroffer, before saying it's well programmed, have a look at the source code. And if your opinion does not change, have a closer look