I don`t think you`ll be happy with running a flail mower half speed. I had to in a pinch, run a 1000 rpm grain vac with a 540 to 1000 converter and it wasn`t pleasant. Maybe the dealer would have a kit or something to change a spocket to it`d run at 540?
I had a 1000 rpm tractor that I ran a 540 stalk chopper for a small amount, ran the tractor slightly above idle, it twisted the 540 converter and i`m sure wasn`t good for the tractor or the chopper
A tractor's PTO (520 or 1000) is matched to the implement it is powering. You can "get by" driving a 540 implement with a tractor's 1000 PTO by operating the tractor at idle . . .but eventually (sooner than later) something is going to break. Besides, a properly matches tractor and implement drive operates at peak efficiency.
Is the mower 1000 rpm only? Some older JD big round balers were made so that you simply removed and flipped the right angle gear box over so that the input shaft was now the output and vice versa. Petty slick actually.