The H2O disassociation temp is 3200degrees Celsius.
Separation methods can use gravity, centrifuging and osmotic filtration.
I think anyone serious about making hydrogen from solar heat would avoid the direct dissociation reaction, and instead go to multi-step thermochemical processes, or hybrid processes using both electricity and heat. Texas Instruments once had a process using microsphere silicon solar cells in combination with a photochemical decomposition reaction. I might be wrong, but I'll give that one a shot here. In the TI process, first, bromine was reacted with water. This is in itself a complicated process, forming both hypobromous and hypobromic acid, which is sort of the same thing that chlorine does when you put it in water.