If you are using v9+ of BigFix / IEM, then you could create a local admin on the endpoint, set its password using a secure parameter, then delete it or otherwise maintain it.
See a set password example using secure parameters here:
The secure parameter uses client mailboxing and encryption. The password is entered into the console, encrypted, then sent to the endpoint so that only the endpoint can decrypt it.
Authenticating P2P sessions with AD credentials is supported and should work, but there is a limitation as you say with Builtin\Administrator in that it only works with local accounts in the system.
I think it would be best to open a PMR and go through the official support process to troubleshoot this issue. Please include in the PMR submission a target log showing the login failure. Here is how to get target logs enabled: http://www-01.ibm.com/support/docview.wss?uid=swg21640296
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