Just asked Christian Persch about this on IRC. His response was:
that's right, all that stuff is not exported, and it's not in any state to be exported. there is no way with librsvg to change the svg without creating a new context and loading the new svg xml into it
If the library doesn't export the stuff on the C level there isn't really a lot you can do at the Vala level. Creating bindings wouldn't be very difficult, but the API that it binds really needs to be public.
Depending on your use case, perhaps you'd be happier using Clutter?