I too have used Qantas Club access to T5 lounges when flying on BA tkt and metal as recentlly as July and August 2001, thoough I was challenged on the August return out of Geneva, which is still a BA (rather than 3rd party) lounge AFAIK.
I would be interested to know if the rules have changed, as not a word from Qantas on the subject, and given the changes to London/Australia BA/QF flights after March next year, this seems very relevant.