You can use $scon(0) to get the number of connections. Then you can use a while loop to go through each connection.

Code:
var %s = $scon(0), %c
while (%s) {
scon -a %s

;Message the channel the timer stuff here.
;You have to decide how you want to tell it what channel to send it to.

dec %s
}


This will loop through each connection, then make sure each connection is active so you can message a channel on it.

You could make an ini file with the channel list in it like this.

file.ini
[Advert]
Network1=#channel1,#channel2,#channel3
Network2=#channel1,#channel2,#channel3

Then you could say:

msg $readini(file.ini,advert,$network) Message...


This is one idea. There are many many ways to do this.