And... success! tbs6984 quad DVB-s working nicely in mythtv under 3.1 kernel on gentoo. And I read the top level readme where it says to how to select the arch to build, which makes all the symlinks. So any problem with the build docs was definitely pebkac.
I was thrown after that for a while that dvbscan could discover the channels, but myth nor szap wouldnt accept the channels.conf that made, but apparently scan has changed the default flags and you now have to specify vdr format output with the -ozap flag for it to work with myth. And once myth has the channels imported, you have to delete them all, then allow myth to scan for them itself.
Whereby it finds all the channels, populates them, and they appear in all our frontends.
I also found I had to push the timeouts right up for channel scan but that may be a facet of my dish signal quality being a bit low.
Happy now, well once Ive edited the 700 surplus channels from the epg