FWIW, I took a look at my own 5x7 B&J just now. There's a counterbore, just slightly larger than the bushing (maybe 3/8 or a bit more), from the top side of the baseplate. I'm guessing the bushing is installed through there - makes sense - if it has a small flange (maybe knurled) it would be impossible to pull out through over tightening the tripod. The bottom of the c'bore is metal, which supports my guess. You might try gingerly tapping at it from the bottom and see if it pushes out (hopefully without too much splintering of wood). I take no responsibility for what happens if you do that though.

Dan