Zoomkat: Your concern about the pigs' welfare is duly noticed. There would not be any sense in using a circuit that was not properly tested. This is however (at least according to me) a given. After thorough testing, I don't see how this robust-looking circuit would fare less well or less securely compared to store-bought ones. After all, I've had both store-bought ones - as well as home-made ones going belly-up on me...
Peter: Your opinion about what constitutes a "good" Arduino project is also noticed. I happen to disagree. I believe a good project happens to be one for which there is the joy of learning and fun in doing, which causes incentive to actually and actively DO. Or I could just say sensible schmensible 
Let's just agree to disagree on this one.
Anyhow, with an added 220uF/35V cap over the primary coil, the circuit works like a charm. There's a definite "bite" when touching the wire. Especially when removing footwear!
I didn't add a cap according to your suggestion. Maybe there will be a price to pay for it, but 2.5 days (so far) running around the clock seems to be working just fine.