Hi Gregoire,
Just buy an Asus SPDIF Adapter ( < 5 Euros at Ebay ) connect 3 cables to the J2 and enjoy the optical output.You should first correct the DVB hardware as lots of receivers don't work with the AC3overDVB, but works perfectly with the bitstreamout plugin (I own one of those).
AC3overDVB already works with the hardware - just use the Patch 0.21 and the adapter as told above.Keeping that hardware limitation (for more info just search the ml...) I think that ac3overDVB is really not a priority, as for example autopid that will works with the hardware ;-)