I have no problem getting MC19 to properly stream multichannel flacs to the Oppo and having the Oppo see them and play them properly.
I did the same thing with MC18.
In both cases, I turned audio transcoding OFF and stereo downmixing OFF in the media server instance.
(I have a BDP-103 and it plays just about everything-audio natively anyway - so transcoding is a complete waste in this case.)
I use the 103 as a renderer, us the JRiver iPad app as controller, and MC19 as server/library. Everything works great except gapless playback.
YMMV
This, of course, involves a DLNA/Streaming solution. If you have a home theater PC running MC (e.g. connected via A/V connections and not network connections to your home entertainment system) then you have to worry about the output capabilities of your HTPC. A lot of audio formats won't even traverse SPDIF optical any more. Too many bits - and no support in the specs. To maximize audio support, you will need a PC with an HDMI output and then run the HDMI out to your home theater system (receiver, pre-pro, etc.). I don't know how well MC works once you have the HDMI out - but I do know that several audio formats are complete non-starters unless you have HDMI for physical transport.