The DUB lounge isn't really a BD lounge, it is operated by Servisair, basically a contract lounge in BD camouflage. I have always found them to be quite sticky about having the BP and the DC Card. While I was waiting for my *G card to arrive the ticket desk had to ring the lounge each time to ensure I got access to the unimaginable luxury and prestige of the BD DUB lounge.
It very much depends who is at the podiums at LHR. Some will flat refuse, others will do it only this once, others will wave you though with a friendly smile. Luck of the draw, Im afraid.
FYI: AC lounges are even more generous. They allow *G family members(adults or children) to be guested in, I think they have to be on the same booking - can't remember the exact rule off hand, but I had no problem guesting my Mother and Father at LAX last summer. Shame the SK/AC lounge is in T3.