I found two problems related to the request pads used by the matroskamux element.

The first problem is that if you create a matroskamux element and add it to a pipeline, but never set the pipeline to playing, gst_matroska_pad_free() is never called and we leak memory. This can be fixed by using gst_collect_pads_add_pad_full() instead of gst_collect_pads_add_pad() and specifying gst_matroska_pad_free() as the destroy notifier.

The second problem is if one actually tries to do the right thing and call gst_element_release_request_pad() on the requested pad from matroskamux. Then this will fail with a warning about an unknown pad. This is because the code in gst_matroska_mux_release_pad() only works for pads which currently have collected data (which of course will not happen if the pipeline hasn't been running...)
gst_matroska_mux_release_pad() should always call gst_collect_pads_remove_pad() and gst_element_remove_pad() for all pads, regardless of whether they've collected any data or not.

The attached patch should fix both of the problems described above.