John,
If you are worried about your dyndns account being updated, have a look at ddclient. There is a setting in ddclient to try and find your external IP address by querying the connection from an external source (I'm testing this mode at the moment with one setup and appears to work fine).
I don't have access to any PPPoA setups to replicate your exact situation so I can't be more specific, sorry.
Kelvin