Excellent! Glad to hear your foray into a DIY transmitter worked out!MWE: Baited breath? What'd you use for bait? (perhaps you meant "bated" breath)
(Everyone else: I know MWE, otherwise I wouldn't tease him publicly.)
The results are in: an AXMEGA32E5 controller in conjunction with an 8-pin Class D audio amp works just fine. I used the controller's on-board DAC to generate the waveform Renbo so graciously discovered and published, and the Class D amp easily drives the 10 ohm resistance of my loop. The amp has short circuit protection. I will use external series resistors to control the width of the detection zone.
I will either layout a PCB or take MWE up on his offer. I probably won't bother with much in the way for surge protection, just a few components, since the cost of this board will be so low. (<$1 for the PCB and a couple of dollars each for the controller and audio amp) I'm targeting a PCB about the size of a USB thumb drive.
Do you happen to know what the model number is for that 8-pin Class-D amp you used is? Sounds like a much simpler approach than what I put together.