Well, I did go on Friday, so here's a bit of an update.
It was raining pretty good, so I didn't get a picture, nor did I drive the tractor again. I did ask about the diff lock though. The salesman said it would stay engaged as long as there was some load on the drivetrain. He also said that I wouldn't need it very often, but I don't know about that. The sticker was not upside down on the one I drove so there is no locking slot. In fact, I couldn't get the lever to move into the locking slot that's there at the top at all. So that seems useless. Maybe it has some application on the gear model?
I did look at the linkage, and it would be simple to modify it. I'm thinking a latch or something would be easy to add, so I went ahead and bought the thing. I'll put some pics up when I get it since there don't seem to be any there now.
Mike