Foster, you said you do "post driving" with your tractors. What are you using to drive the posts? Is it a post pounder that is driven by a PTO pump? When you are using the driver, are you on the tractor or standing at the rear to run the post driver? Where you are and where the pump can be located is a big consideration. I personally think you will become very tired while pumping the manual pump.
The solution Soundguy suggested seems the best. Perhaps you could build a totally portable toplink and pump that could be moved between tractors with an easy electrical connection something like jumper cables. Finding a place to put the pump and reservoir on different tractors might be a challenge too. The more money and time you spend designing the setup, the more convenient it will be. A cheap solution will probably be a pain and not save you that much labor.