I am not aware of any such convention, formal or informal, and if one did exist i would think it would operate at the 1-2% of threshhold vs 5%.

Since you are qualifying on points you need spend vs nights or stays. If you would otherwise do any holiday hosting you could try to do it at a Hilton property and bounce for a room to aggregate the spend to a folio and HH credit- plenty of such options here in London.