This is virtually impossible and if it was possible very much not recommended. I would pay to reserve plaza premium in t5 or get dragon pass/PP and access that lounge or Aspire t5. Plaza premium is the same as you would get in to anyway I think? QR use that for their FF IIRC. You could also spend the money in a restaurant/bar and I say that as someone who always wants to visit the lounge! The other option is pushing your status to sapphire before this trip or upgrade your ticket to Club Europe on BA (which could help you reach sapphire? Not sure the QR rules on cash upgrades).