It can be done with a MiniDSP DDRC-88D. The trade-off is having to disable MHR/HS but, for me at least, the sonic benefits of room EQ across all speakers outweighs the benefits that MHR/HS provides.

I have also considered placing the MiniDSP between the HD621 and the M1 inputs but I've yet to find an audio solution that hooks up to HDMI and I quite like my Squeezebox Touch still.

Chris
Srces: Squeezebox Touch | OSMC Vero 4K+
Procs: HD621 > G61R > Minidsp DDRC-88D
Spkrs: 7200.2 + 5KC + 5Ks | iNuke6K > USC15 | iNuke3K > 2 x Crowson T108SS
Video: JVC HD950 & 1.9m React3 2.4:1 screen
Room: 5.8x2.8m | Various GIK Panels