For a Premium user with a Green Card, maybe it could be made in such a way that the Online Cards automatically get "upgraded" in a way that you can also collect trees with them. Hard to say exactly what is the most realistic approach without having data on how much money bunq actually gets from interchange fees. Right now the Online Cards use the same BINs as the physical Mastercard Debit, so they probably also are MC Debits cards, and not Credit.
If my assumption is right (it doesn't have to be), that only the MC Credit card interchange is enough for bunq to finance the tree planting project, it would be nice to be able to also collect trees with your Travel Card(s) and maybe a new version of Online Cards that is MC Credit-based, as long as you also are a Green Card user. Or, well, a special "online green card" number would also be one solution.