folding stem
Charles, the steerer tube is that which came with the bike - big, fat and thick-wall aluminium, made to fit the frame's 1 1/8" head tube, and too fat to accommodate the boom clamp. I had a local machine shop remove the portion below the hinge and re-weld a smaller diameter tube. The guy did a gorgeous welding job. I haven't taken the bike out yet, but have pushed and pulled and yanked on the handlebars a dozen different ways and I don't detect any flex. As you can see, the hinge is substantial and the knob can be cranked down really hard.
I agree with John's observations, and the Terracycle GlideFlex solution I had proposed would likely have had issues. With a ratcheting clamp below, and an adjustable headset on top, there would be too much flex and two potential points of failure. I can only hope that what I have now is strong and flex-free enough to take it. But, honestly, other than when I'm climbing and pulling myself forward, it doesn't occur to me that I'm putting that much pressure on the handlebars.
Because I wanted to keep the option of reverting the frame back to a conventional folding bike, I ordered a second steerer tube for the bike, knowing I'd be forever altering the original for this project. I'm pretty confident that I could ask the SpeedOne folks if they could make more available. I paid $28 USD plus shipping.
I'll keep you posted regarding riding and handling characteristics. The best news will be that it handles as before.