Well, the boulder put up a good fight, but finally lost today.
I went back to it this a.m. with more hand tools, including a heavy chain. No luck with the chain -- tractor just didn't have enough traction in the still-damp ground (mud).
So I just started digging alternately with FEL, pick and shovel. Finally got so I could tilt the beast up a little bit by pushing and curling the bucket. Then while the loader held the rock I hand-shoveled dirt back into the hole behind it so it couldn't roll all the way back into hole, and repeated this cycle a few times, each time leaving the rock a little higher.
Finally it was up and out far enough that I could push with the bucket and get it rolling downhill and off the trail.
I found a "sister" boulder sticking up nearby, and it wouldn't budge. I'm leaving it where it is and will mark it with a driveway marker so I remember where it is and avoid it with the Bush Hog. The Kubota has earned a rest anyway.