It is already easily doable. You add a dummy connection, and use dbus-scripts to run the script that connects to pptp when this dummy network is connected, and run the disconnection script when this dummy network is disconnected. I used to do it on N800/N810 when BGU's network used pptp, and I do it now on N900 for bluetooth DUN client connection.

It is a pity that Nokia decided to make dummy network module such a pain to get, but there is an almost compatible free replacement.

You can see the bluetooth-dun-client package on my repository for an example of how it is done.