Have a look at the Hotter range of sandals as some of these will take insoles, and some are more like open toed shoes that give some ankle support. They are not cheap, but you can pick them up on ebay and in sales.
Diagnosed HEDS December 1st 2005. DD1 (20) HEDS and scoliosis (now corrected by surgery), diagnosed June 2006. DD2 (18) mild HMS. Son (11) some hypermobile joints, poor muscle strength and seems to be developing scoliosis as well, woopee!