I've owned and restored three steel Boatel's over the past 9 years. Not every carrier demands a ping test but most are moving that way. I've heard a couple of folks tell me recently that they had no problem getting insurance with just a thorough survey, one said they used Progressive and another State Farm. I find that hard to believe but you could try. I get to talk to lots of folks on this issue because I'm on the board at the Lake Powell Yacht Club in Page and our yard is filled with mostly old steel boats. Problem is the data shows that the old steel boats are causing the carries losses so they are on the lookout; The newest steel boats are 30 years old. I'm a huge Boatel fan but they are starting to disappear from the lake, every year there are a few less. If anyone has a boat they need to work on the Yacht Club has space, is open 24/7 and there are toilets and showers. You can stay on your boat there while you work on it, we are a group of active boating owners inside the storage yard, We are a non profit so our rates are less than retail. It's close in town, by Page High, that makes hardware runs easier.
When I was asked a few years ago by Warner/Volk there in Page for a ping test on an old aluminum boat I was shocked at the cost. Being a do-it-yourself'er to the extreme I asked the agent if I could, well, do it myself. The agent checked with the carrier and they said I could. Meters ranged from $200 to 2,000. In my business I use lots of professional grade meters and write reports so doing my own ping test don't seem out of the question and I wouldn't buy the ultimate professional quality meter to use it once. Hopefully they would accept it in the amateurs form I'd submit my report in, IDK, I went another way instead so I'll never know. The ping test was requested for full coverage, I took liability only.
In my opinion It is incumbent on old steel boat owners to be honest with starry eyed dreamers buying their first "cheap houseboat." To not fully disclose to them what you know about the insurance market wouldn't be right. As long as you did that it is their problem but you're a good seller for trying to help them out either way.
I have good luck on insurance with Karl Norgen in St George. He used to live in Page, used to office with Bull Dog Boat Repairs. He's a boater too, he brokers American national but that may not help your folks if they tried them already.