avast doesn't dial out period, so you would need to schedule something else to connect that does dial out. Then avast will see you have a connection and check for updates, if not it won't try to update. This solution is very messy and I suggest you let avast take care of your updates during your daily browsing, etc.
avast uses a cumulative update process so it is very fast and there is no real need to do it at night. I also have dial-up and shortly after I connect to the internet avast checks for updates and notifies me (pop-up notice) an update is available. I click the notification window if I want to do it then, otherwise I do it manually later but mostly then and there. I have my updates set to notify but not automatically download and install, which I prefer, especially on dial-up with the possibility of early disconnects/dropped connections.
