If the "[+]" was actually a treeview control toggle button, then you can pretty much forget about it. The buffer windows work through manual drawing of each character in the window, implementing dialog controls inside windows would mean Khaled would have to reinvent the wheel on dialog control drawing as well (not to mention the entire API for handling the interaction events)-- I'm fairly sure that's not going to happen just for one ignore feature request.
If the "[+]" is just an ASCII representation as I imagined it was, you could easily trigger a hover/click through a HOTLINK event. In this case, all mIRC needs is the ability to update channel buffers the way it does with @windows (through /iline, etc.)-- but this would also be a very big change, I imagine; perhaps a little more likely to happen at some point in the future though.
On that note, if the OP wrote a script to display channel info through custom @windows like some theme scripts already do, he could accomplish this through the method I described above. Of course using the colour code method is much easier and no less intuitive to use. In short, you might not get some treeview toggle control button, but there are various ways to do something almost equivalent in mIRC right now, and given how few people have requested this kind of a feature, I'd say that having any kind of existing reasonable solution should be good enough.
