Got a call back today from the underground pipe locater guy and spoke to him about my issue. He said that in order to find that plastic pipe underground, he would need to be able to send either water (preferable) or air (less preferable) though the line in order to detect it. But since one end is open, and under the house, that would have to either get capped or have a hose bib installed on it so we can do what we need to do. That would be the first order of business. And, he said his rate is $125 per hour, with a two hour minimum.
The idea of spending time under the house to get that galvanized end cleaned up and either capped or a valve installed, the idea of getting a bit of help with it crossed my mind. I looked to craigslist under the skilled labor section for plumbing. Called one outfit that seemed reputable, but they wanted $26 just for showing up to look and estimate. Found another guy who is not a licensed contractor but has worked in the trade for a while, and who calls himself semi-retired.
He came by and I took him around, sharing my issue with him. We talked for a good 45 minutes and finally concluded that capping that end under the house is risky since you cannot guarantee a permanent, leak free seal. If it were ever to leak, you would not notice it for years, perhaps. Best thing, still, is to keep the water from coming back under the house.
That left one of two main options. I could either keep digging, looking for that line, or I could abandon it, and run a new pipe along the fence back out to the yard and garden. The latter idea started to gain traction in my mind. The new line wouldn't need to be buried that deep at all, and painted black, it could handle any UV rays pretty easily. This would mean no more fruitless digging, and completely and absolutely bypassing the house.
This now appears to be my way out. It's more work, but at least the work that goes into it will be positive, every ounce toward the goal. Quick look at the family calendar for this weekend shows that I have all weekend. Maybe it will get done, once and for all and I can start my spring planting!