add-on for domain registrations -- any work-around?

I just also posted this (below) to someone's 'feature request'.
But maybe you have an idea for a work-around -- how to add an "add-on service" to domain registrations during the order process? It does not have to be automated in the sense that this would be handed on the the registry. Just so that the payment is being made with the original order (for those who need this add-on, "trustee service" in our case).

We have many international customers who want to register European TLDs such as .fr or .se and other TLDs with various locality conditions (such as passport numbers, citizenship, etc.). We can often offer them a "trustee service," but for a fee, sometimes a relatively high fee (higher than the actual registration). If we could associate an add-on to registrations, then, in this example, an American customer could add "trustee service" to his .fr domain registration, and we could automatically provide the registration to him. OTHERWISE he (as it still is) cannot order a .fr domain, or only as a custom order with manual setup (a lot of back-and-forth, a lot of work).


Best,
John