I may be off the wall here, but is it possible to use something like a 568 (for 24/96) to process the side channels of 7.1, using a G68 or 861 for the others? The feed would be from the fourth SPDIF of the HD621, assuming this were enabled.
I suppose there might be problems, but it is not obvious to me exactly what they would be.
Would be relatively cheap if it could be made to work!

Locus