Plug an exciter into the center channel, and stick it to your backbox.
I am going to guess that since VPX inits bass.dll before vpinmame is called
and because the bass.dll that vpx 10.7 has is fully capable of using 7.1 channels for sounds designated as backglass
(yes you can create 8 channel ogg's for backglass sound effects)
and it is already in memory
I am guessing that vpinmame simply uses the bass.dll that is already loaded, rather than calling up the lesser version that it has
which would then make it address the sound system at it's full capability, instead of 2 channel stereo only.
That is my guess anyways
I may be the only person presently who has a table that has backglass sound effects in DTS 8 channel (And it isnt even a terribly good table as far as pinball goes)
but i imagine that eventually, someone will go and do a good original table, and go and give it dolby surround backglass sound effects.
At least you know you can shove an exciter in the backbox some place for the center tracks to come out of 
Could put an actual speaker too, but that may be a wee bit of a pain to do on an already build cab