Will an E-ATX Board fit in the new Corsair Obsidian500D

Well if you know where the mobo and case holes are, you can estimate if you'll need them all (i.e. the right edge in the picture above), but it would suggest that the board will overlap the cable management holes.

For what it's worth there are only three 'completed builds' on PC Partpicker, and none have E-ATX boards. Early days.