This may not be possible, but I do have this working in both Foobar2000 using MatrixMixer component, and in Audition using the built-in Audio Channel Mapping.
I have FLAC files which are 8 channels. They are made from 5.1.4 ATMOS file decodes, from the Dolby Reference Player. I have mixed the center channel into the front channels to create a phantom center, and I have discarded the sub channel. That allows me to go from 10 channels down to 8. The layout is 4.0.4 rather than 5.1.4.
FILE LAYOUT
1) Left------------1
2) Right-----------2
3) Surround Left---5
4) Surround Right--6
5) Top Front Left--7
6) Top Front Right-8
7) Top Rear Left---5
8.) Top Rear Right-6
.....................................
Speaker Layout (No Top Rear Speakers, must redirect to Side Speakers)
1) Left
2) Right
3) Center (ignore this channel)
4) Sub (ignore this channel)
5) Surround Left
6) Surround Right
7) Top Front Left
8.) Top Front Right
