Why not add a simple public page to the Tarsnap website that allows the visitor to gift x dollars to the foo@example.com account? That way, bitcoiners can use bitspend.net etc. as a proxy payment provider for the time being.

Not leaking metadata on credit card statements that allows to infer that valuable data is stored at Tarsnap (and approximately how much), would be a practical increase in security: Such a leak might prompt an attacker to allocate more resources towards compromising the Tarsnap user's client computer.