When you use the block of wood method, the lathe is in the slowest speed, and you just bump the motor switch so there is no fast spinning, and little danger of the chuck falling down on the ways.I have 2 10x24s one is a Jet 10x24 PY, Yes the chucks can be stuck with problems , one way was to ensure the belts were tight, Then put it where it takes the most to turn it (forgot rather it is high or low speed). Then put a long bar in your chuck jaws and tap hard with a rubber mallet.
this may be enough to make chuck turn loose. They are screw on chucks, In fact I am also looking for backing plates for mine.
It is approx. 2 inch with 8tpi. It is not really recommended to lock it with the back gear, as you can break teeth on the gears if not careful.
The long bar has work for me a couple of times. But you also have to make sure you do not mess up our chuck jaws.
One of the problems I see with doing the block of wood on the back of the ways, and powering I reverse is it will quickly spin off and crash down on the ways. Or at least that is my thought.