Hi all, I'm pleased to report that this issue has been resolved in Intel AMT SDK 8.0, where you can now pass the default admin user name and calculated Digest Master Password credentials through to establish a Serial Over LAN session.
On a personal note, I'm also pleased to report that the Intel team do take issues we come accross serious and are actively improving Intel vPro and associated tool sets. As with this problem I was facing, it was esculated to the Engineering/Developers team and within a few days they resolved the issue and released a new version of the SDK.
So, thank you to the team at Intel!!