I am pretty sure you will be able to control the PPP independently of the sync in the interface.
You can also use the billion in bridge mode (like I do) so its then just a replacement as a modem for the hg612.
ON my 8800nl, I have done lots of fiddling on vlan settings, bridging settings, firewall, ip range, wireless settings, snmp and more, none affected the sync status. The only things I expect that would initiate a new sync is directly touching the dsl settings.