Thanks for the suggestions, gentlemen.
I also tried the profile manager which only allows assignment to walls, beams and columns.
You can rotate beams on their axis and angle the beam for 2 dimensions, then rotate on plan for a 3rd.
Theoretically I could go around the roof with small lengths and adjust the position of each one, possibly to tie into the short lengths represented by the barrel vault but this would take ages.
I was able to use a spiral object from 'basic shapes' to give me an idea of how it would look.
It's not accurate enough but it has highlighted that the idea is simply going to be very difficult and expensive to build so I've ruled out the crazy 3D curving spiraling soffit...
I thought of modeling the co-ordinates in Solidworks or ProDesktop and importing an object but this is the only instance archiCAD (and I) haven't been able to deliver in 15 years so i'm quite happy to continue producing architecture without spirals!
Thanks again for all the advice!