Not many answers, but I thought I'd go ahead and update it, before I forgot.
I did confront Seth Monday evening. He and I reviewed the original deal we made, agreed he hadn't been keeping his commitments, and I made him remind me of what his dad had said about late fees.
We were in my office, in private, so I had Seth remove his jeans, then fetch the large stool. I took a seat, and had him climb over my lap.
If you don't remember me talking about this bar stool before, it's because I don't use it often. Unlike a chair, where the seat is 18 or 20 inches off the ground (about knee height on me), I keep the stool set about waist high. Even a boy like Seth, who's as tall as I am, has to use the foot rest to get into place, and then their hands and feet are dangling in the air, just like happens in a normal chair, with younger boys.
I'm sure he was in an interesting place, since I'd let him keep his boxers on, but my hand was resting on his bottom, and he was in a very vulnerable, juvenile position.
I asked him questions and lectured, giving a swat or two in between. Since he wanted to give me a lot of excuses, those smacks started to add up.
We came to the conclusion that, while he does have a number of things going on, he'd given his word, and keeping his word and paying off his debt were more important than anything (except arguably school work and band, though he should have considered those things when making the agreement).
Seth still owes me 20 hours of work Finals are coming up in four weeks, but he's a good student, and those shouldn't have much effect. He also has marching band practice after school each day. We agreed that he owes me at least two hours of work each week between now and the end of school, then six hours until it's paid off. Any week he doesn't fulfill that, he gets a spanking (I was very vague, so I can make it anything from hand spanks to a real paddling, depending on how he's doing otherwise).
I finished with a quick flurry of hand swats - just enough to have him squirming and yelping a bit, but no where near a 'real' spanking, which is why this doesn't show up on the Spankorama.
When I let Seth up, he took a moment to rub, but apologized while doing it, and then I got a big hug, so I'm pretty sure we're okay.