As a workaround, you might try to edit the file ./usr/bin/db-orange, and add the line
dbus-send --system --dest=com.nokia.icd /com/nokia/icd_ui com.nokia.icd_ui.disconnect boolean:true
After the pppd call ... line.
This will disconnect whenever pppd dies, which is what I expect to happen when bluetooth gets out of range.

EDIT: This will obviously not work. pppd goes to the background and returns immediately.