Personally I think it should be per wallet address. If people have multiple wallets there is no reason not to hold in one address. It is called bag holding ! Majority of people have one address.

But in any case, if people want to have multiple wallets, they can. They will still get paid but may miss out on the bag holder bonus's.

However, if it was voted on, a back end to the script could be created that would allow you to join wallet address's.

You could also add the ability to add a bitcoin address linked to each qbk address. Again that would be a back end thing that only administrators would have access too.