Richard, having discussed this with Dan a few times over the last 12 months, he knows the issues with mine (1st production 5link) problem is the spring length available on the MMC setup is difficult to get 2 coil springs in and not coil bind in compression, but fingers crossed he develops a solution
Yes I mentioned to him the possibility of concentric springs to achieve desired progression but don't know if that's practical. His follow up suggested not. Apparently a single progressive spring wouldn't give the necessary roll resistance so final solution will be something of a balancing act.