The beauty of Tarsnap is to provide a tar-like command-line tool. Here, while the later goal is to provide storage in a more generally accessible way, the beauty is to use scp as-is, which you already have (if you're a developer).

A client-side encryption tool could be implemented and use any scp accessible hosts, but that would not be really a Tarsnap equivalent: there is still the problem of avoiding redundant storage and traffic. Bup could be a solution here.