In all honesty Rob, I think forking out the hard earned on a set of Cole Jaws is the best solution in the long term.
I have made longworth chucks, home made cole jaws etc. in the past and all had their limitations.
As you say getting a longworth chuck to clamp with sufficient pressure to turn something like the segment ring is neigh impossible, fine for cleaning a bowl base up with tail stock support but that's about it.
Home made Cole Jaws, unless you have the means of drilling the fixing holes with the accuracy of a dividing head etc. much the same limitations on run out but with better griping power.
Cole jaws can also provide a good (adjustable face plate) clamping base for odd shaped pieces with the use of some threaded rod, long bolts etc.