Has anyone ever had any luck getting threads started on a cut-down steerer? I know it's not the recommended procedure, but it wasn't my decision; it's just where I am right now.

you need to takes to your LBS and have them do it . hoply they have the tools for the job . use a file and put 45 degree on the edge then you need a threads cutter tool ( like park tools make ) to cut the threads . turn the tool half way then back off by a 1/4 then 1/2 and keep doing this until you reach your limit . (your ending point ).

Have someone spot you to help start it strait. Use lube.

I'm definitely taking this one to a shop. I don't have the tool and at $200 I don't intend to. I had heard that trying to start with no threads is a nightmare but after several calls I found a shop that's willing to try. There's not a lot to lose - it's an old Schwinn fork I found at the LBS for $12. The main issue is I'd rather not have to look for another one.

I feel sorry for the boss of the kid that's going to try. Probably going to destroy the tool and your fork.

You could take it to a machinist who could do it with a lathe, but that will likely cost you more than the price of a new fork.

If I were you, I'd just buy a new fork that is the proper size.

My LBS acts like it is no big deal. He did one for me. Only cost a few dollars and did not seem to take long. What am I missing here?

Steerer threads have to have some precision to ensure the headset bearings engage with their races evenly and completely. Steerer tube taps are designed to extend existing threads - not start new ones on an unthreaded tube. Trying to maintain perfect squareness while starting to cut new threads is very important, extremely difficult, and very time & energy consuming.
But hey, it might work out. I'm just giving my opinion here.

That's consistent with what I've been told, but I'm hoping to get lucky. The guys at the shop where I dropped it off today weren't newbs and said sometimes it doesn't work out but they'd try. We'll see.

Follow-up: starting new threads failed. The shop managed to cut some threads but they weren't straight. (This was the only shop out of 3 or 4 I called that would even attempt the job.) Extending the existing threads before cutting definitely seems like the way to go. I just really wish the first shop had followed that rule, as it was a pretty nice fork, and these puppies aren't so easy to find, I've discovered.