You might take a look at the bottom of the tractor about in the middle just behind where the front axle driveshaft comes out.. There is a bolt that looks like it might be a drain plug, except that it has a hole through the center that has a cotter pin through it. If hydraulic oil is dripping from that bolt which is a drain, it indicates a leak from the hydrostat unit which is housed inside that casting. The leak is probably a hydrostat seal or possibly (slim chance) just an o-ring on a line attached to the hydrostat. A leaking o-ring should be detectable by leaking oil migrating back on the oil line. The bad news is that if the oil is leaking from the hydrostat unit, it is likely a shaft seal and the tractor needs to be split to address. Also, if it is a hydrostat seal, it will likely get worse.
The drain bolt with the cotter pin can be seen at the bottom of this photo;
I believe that bolt/cotter pin is meant to be a drain for water and or oil that gets into that area. Water can easily get in there so it is good to check that the drain does not get plugged over time. I reach under there and spin the cotter pin around a few times at least twice a year to keep it clear.
Some newer Shibaura built models may not have the threaded plug and there may just be a hole in the casting with a cotter pin in it. Also, there may be a hole with a cotter pin further forward under the clutch. That would drain any oil seepage from a main seal leak on the engine. You are looking for the rear most cotter pin.
Edit: Rereading OP, if the leak is power steering fluid, a leak at the hydraulic steering valve at the base of the steering column could easily
find its way down to the same drain hole. There are openings in the top of the casting for oil lines and transmission linkage for oil leaking from the area of the steering motor pass through.
A leak around the orbital steering valve should not require the tractor to be split.
A word of caution about working around the steering valve. Cover the openings in the top of the casting where lines and linkage pass through with rags or tape. You do not want to have objects drop down in there.
