I'm using proxy only without NAT/Masquerading and i'm able to deactivate the PPPoE-Interface without any problems.
But i remember this discussion a few weeks ago.
If you want to simple connect/disconnect the WAN-Connection (due to a time-based rate) Astaro isn't the right one. You should try IPcop or Smoothwall. They have a simple Connect-Disconnect page in the Web-Interface.
I think, they don't disable the WAN-Interface completely. If i do this in Astaro, it will take several seconds, if the interface is down. On Smoothwall it's about a second.