Well, I gave in and took a Sawzall to it this afternoon.
A few minutes and two blades later, I had it on the ground. I knew as soon as I'd actually breached the pipe because water started pouring out-something I hadn't counted on. I just kept going when that happened since I figured it would help keep the blade cool and clear.
Once most of the way through, I used a crow bar to open up the cut a bit, then bent it down to the ground, back up, and about 3/4 of the way up it broke off.
Assuming it quits raining, I'm headed back out tomorrow for a bit more clean up work. The pole probably needs to be cut in half for easier disposal. More importantly, I didn't quite get it flush with the cement, especially since some cracked off while I was working today(it's rained in the day and gone below freezing at night a few times over the past week, so I expect I've had some help breaking up the concrete). Consequently, I have a nice jagged edge left over, and I'm thinking it might be prudent to beat that in as much as I can with a sledgehammer before burying it again. Then, I'll call it done.
BTW, I don't know how long I'd have been digging, but my tape measure bottomed out at 3 1/2 ft below where I cut it...