Hi Russ, yes Hyd pressure is all that keeps it clamped, dont have any monitoring of the pressure but there are limits, or will be, that look at the clamped and unclamped positions so if though necessary these could be used to throw an error.
People have made their own couplings up but on a small machine the dowel would be good enough I think, as Dan says tapered pins are the way to go and you can readily get tapered dowel pins and tapered reamers to suit. If going that route it may be a good idea to actually have the pin move via air/hyd/motor so that the turret plate itself doesnt need to unclamp.
Hood