After more discussions with mtnbikebend on mtbr.
http://forums.mtbr.com/27-5-650b/chinese-carbon-frames-650b-edition-820784-15.html#post11082757It looks it is relatively easy to pass the cables for the rear derailleur.
The bottom hatch below the BB looks to be there to help on this.
You can re-route the cable through the frame without using the housing that came with.
And crossing the cable in front of the bar, to get a nicer bend is ok too.
In addition he used the ESI grips, and cut them to the right length to cope with the Gripshift and have the brakes at the right place.
All this is obviously still theory for me, but in a couple of weeks, I'll try!