Hey there, one of the diffscuss authors here.

We were working for a client who used self-hosted svn and had no code review system.

As for GitHub API: there's a (beta-ish) import from a pull request into a diffscuss file ("diffscuss github-import" for more), but currently nothing to push it back, which is a tricky problem (since what's displayed on a github pull request is actually a mixture of pull request comments and comments made directly on the commit diffs). But if there's sufficient interest we'll take a look!