So, I did a few experiments with gstreamer, but without much success. But maybe someone finds this useful, though.
It seems however like rockbox is using gstreamer, and has "pulsesink" as output sink hardcoded (http://git.rockbox.org/?p=rockbox.gi...cm-gstreamer.c), so one could try to change this and recompile (or ask the author to make it configurable somehow).