I was alluding to the quote from the 'Ulimate BA Guide' under lounge access:
'7. Lounge access for connecting passengers
....
British Airways lounge access also applies when travelling inbound in First Class or Business Class on a oneworld carrier.
For example: Travel Chicago to London Heathrow on AA in First Class arriving LHR T3, departing on BA ex LHR T1 to Stuttgart in Euro Traveller. Passenger is entitled to use the BA First lounge upon production of their AA boarding stub and onward BA boarding card'
Doesn't this mean that if the Premium Lounge operates under BA rules, at least for BA passengers, then I would be allowed access (with the usual one guest)? But if it operates under AA rules, then I would not allowed. The problems is it is a joint AA, BA & IB lounge. Which rules apply? Perhaps I should ask BA to have something in writing in advance?