I just reread your post. A script going on the address would be alot better. So here it is.

Code:
on !*:JOIN:#channel:{
  if !$($+(%,$wildsite,.,welcomed,.,$cid),2) {
    set -u43200 $+(%,$wildsite,.,welcomed,.,$cid) 1
    msg $nick 05Hi $nick $+ , this is a message that you will receive only once every 12 hours.
  }
}