I'm curious. I can't see any functionality in librsvg for loading an
SVG DOM directly without doing a round-trip into a string. Am I
missing something?

Good point. I hope the following will perform better.

RsvgHandle *
rsvg_handle_new_from_stream_sync (GInputStream *input_stream,
                                  GFile *base_file,
                                  RsvgHandleFlags flags,
                                  GCancellable *cancellable,
                                  GError **error);