When I had my BX22, I installed rear remotes by hooking into the 2 back hoe lines, and added a return to tank from the valve. Unfortunately, that valve developed an internal problem and no repair parts were available for it, so I removed it from the tractor, since it was messing up the hydraulic system of the tractor, and the loader wouldn't raise.
I thought about doing the same with the BX23S, however, there isn't a plug in the side of the transmission to install a return to tank line. I started installing a Land Pride 3rd function valve last weekend, and had the rear wheel off of the tractor and was looking for a metal plug, but there was none.
I would like to use the 3rd function valve to power the backhoe thumb, but I don't know exactly what size cylinder that I will need. I have been looking for more information on this, but so far, I haven't found anyone that has done this other than Paul Short, and he doesn't give very much detail other than to say that the cylinder that he used came from Princess Auto. Since there are no Princess Auto in the States, that is of no help.