So it turns out my assumptions were wrong and the format negotiation is part of the eARC communications channel, not CEC. (Although I was correct that it defaults to a failsafe mode supporting basic stereo formats.) I now have working 7.1 PCM capture over eARC! 
Here is the magic incantation to report 7.1 PCM support (with all sample and bit rates):
amixer cset numid=6,iface=MIXER,name='eARC_RX CDS' 0x01,0x01,0x08,0x23,0x0f,0x7f,0x07,0x83,0x0f,0x40,0x00,0x00
Once this is set, it ‘just works’!
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