I saw a guy out yesterday with a sail attached looked like fun, the only problem was they took the sail out and started catching waves until the guy fell off and guess what no leggie it nearly cleaned everyone up
I think all SUPs work well as windsurfers, in the same way (non-planing, goes upwind more with the windward rails than the fins, but do not plane frankly due to the rocker). So basically chose your board for its SUPing caracteristics first, it will windSUP fine.
The only ones that windSUP differently are the ones specifically for sailing first, like the 7'6" Sealion. Their lack of tail rocker, huge tail, short length makes them actually zip on a plane like a windsurfer , but then their SUPing suffers, they are trickier to handle (although fun in small slow waves like a fish) .
I use a 9'8" starboard fish for both SUP and Light wind sailing when there are waves and 10-15knt cross off - works a treat at both and will plane with enough wind and sail - they slam gybe - easy tacking and are pretty good fun on a wave. Mast track forward position seems to work best as they tend to ride nose high otherwise and spin out on bottom turns easier if mast base too far back
As far as a leg rope goes on a windsurfer - (earlier comment) who are you kidding you cannot put a leggie when windsurfing unless you want to get fully tangled round the mast base etc.
don't windsup where their are heaps of surfers out is probably the best option unless your very confident as these f&&kers can kill someone if you hit them in the head.
it nearly cleaned everyone up 
