"Could you add a minize key (I propose Escape) that'll minimize mIrc (to the tray)?"
Good idea, there might be some key combination that already does it that I don't know about though..if nobody posts one, there probably isn't

"And could you add an option that'll always keep mIrc out of the taskbar (and in the tray) like ICQ and WinAmp have?"
I think what you want here is already in mIRC:
ALT+O > Display > Options > [Tray]Check the box
"Place mIRC in tray when minimized".
Hope this helps

Regards,