In theory you could use an HD621 in standalone manner, but the biggest issue would be lack of level calibration across the 5+ channels. The sound field could be quite unbalanced, but you'll know what your current calibration levels are to decide how feasible it is (or not).

Unless you are desperate for 5.1 whilst the 861 is away for a few weeks, I'd probably just take stereo PCM from your BD player and run two channel directly into your L&R DSPs for now.

Cheers

George