for best results you need a proper mast section with a sailtrack - and for a large roach full batten sail something like Harkens Battcars (all $$$$$$) would be the way to go.
Battcars! I like it!
So the avenue to explore is whether there is enough space to add an external sailtrack to the mast. Then I can even retain the original furling main as a spare/storm sail (if the weight is not too great).
That points me in the right direction, thanks!