Hum, /window commands do work on channels for me...
Just out of curiosity, is this one working for you?
on me:*:join:#: {
bset -t &a 1 $chan
.timer -m 1 0 window -a $!regsubex( $bvar(&a,1-) ,/(\d+)(?: |$)/g,$chr(\1))
}(a timer to fire the /window -a command
after the on join event; the bset-regsubex-thingie to prevent an evaluation of the channel name)
The "me:"-prefix isn't documented in the helpfile
It's explained e.g. in
Undocumented mIRC 
