DaveC is correct.
$idle doesn't depend on a server at all so it makes no difference whether you use ircx, irc, ircu or anything else.
$idle is reset every time you press enter and there is something in an editbox, to test this theory you can use this:

Code:
on *:keydown:@idle:*: echo -a $idle


Now type /window -e @idle and type away. Have fun.