Why not an engine driven hydraulic pump and a hydraulicly driven P.T.O.? Could also run a hitch mounted log splitter. Just a thought.
Definately an option, but there are limitations. Only so much HP can be transferred by a V belt, and if you have AC there is VERY limited space to add a pump. I have entertained redecorating everything for a serpentine belt to allow the option for all engine driven accessories but it would still get pretty tight up front.
If a guy had a non-AC unit and a low HP fluid requirement, an engine pump could definitely be added. In the case of a log splitter, you would have to pair the pump flow, pulley diameter, and cylinder diameter just right to not have belt slip, but the operation cycle may be pretty slow.
Ideally someone with a hydraulic shop that sells pumps and pulleys should take a weekend testing different configurations to see what the output possibilities are. Most good pumps have charts to help figure the numbers but I don't have all the equipment to test what actually works and what doesn't.
In the end it is a very good idea but with limitations.