Yeah Scott, probably 3/4" ply would crown that much without kerfing it on the back. I'd put a bead of construction adhesive on the beams though, so that the screws aren't the only thing holding it in place (one wouldn't want anything to suddenly go "sproing!"
I'd screw down the center of the panel first, and gently encourage the bend into the ply by putting in a line of screws 6" from the center on one side, then 6" from the center on the other ... and so on out to the end of the plywood sheet. Don't be tempted to use drywall screws - they're brittle! I'd use deck screws, as they've got a rust-resistant coating ... long enough screws to go about 1" into the underlying beams.
That said, if the panel does turn out to be too stiff to bend, most 3/4" I've seen has 5 plies. As you suggest, you could set your circular saw at a depth to cut through the first two, and kerf it from end to end every 4" or so on the back to add some more flex. Do it before you stain it, and slop lots of stain into each of those cuts before the thing's screwed into place, or you're inviting rot.
Since the platform will be laminated from two or more thicknesses of 3/4" plywood, that kerfing won't have any material effect on the strength of the finished platform so long as the glue bond between the sheets of plywood is substantial. I'd use lots of a premium polyurethane construction adhesive - it's as strong as epoxy and more flexible - so you won't shatter the glue line when you drop weights on the platform. Waterproof too. But hell to get off your hands ...