I've received this specific advice from legendary ultra-distance inline skater Eddie Matzger, who said that this very training has saved his hide many, many times.

From my own experience skating and teaching skating (inline, not board), one of the easiest ways to get injured in a fall is the "stick your hand out" reflex. Fall training mitigates this, ingraining habits to allow much larger areas of the body to take the impact, and ideally take it rolling. If you're going forward (doing the "superman") and have wrist guards or palm sliders, arms out can be ok. If you end up going more down than out (esp. many backward/side falls), it's likely a broken wrist or arm, right through any guard you might be wearing. (ask me how I know that one...)

Long time street-skater here (30+ years). Not sure that stuff would help as it's pretty different rolling around in a gym vs. getting thrown off your board at 20mph. It also might actually make you ride over your head because you think you can bail and not get hurt. I also don't think pro skaters (inlines/etc...) are good examples because their skill level is already so high; they are operating on a totally different level. They already know how to fall really well because they've fallen a ton. Also, if you are worried about getting hurt, you probably shouldn't ride a skateboard.

The best preparation is just to get a regular skateboard (not a long board) and skate around on it on the sidewalk. This will help you get the balance and board handling down.

I highly recommend learning how to ollie on the regular board. The falling you do in learning how to ollie will help. Even if you can't ollie a boosted board, the board control you learn will help you manage riding over rough pavement. Also the ollie motion of shifting your weight can be used even on long boards to manage obstacles.

Finally, on the regular skateboard learn how to power slide. This will teach you how to balance your weight if the board is sliding sideways.

I did some adult parkour classes. We learned how to break our landings outside on concrete.

No reason you can't take your gym training outside and start falling on hard surfaces.

Would it help to wear full motorcycle-type gear?

This is exactly what many downhill longboarders do. Typically with suits made by places that do motorcycle racing leathers. It's absolutely not a substitute for fall training, however. See my earlier post in this subthread.

Probably, though I'd guess the additional gear would be at odds with what I suspect is an appreciation of minimalism in the product.

While it might help, you will be falling to much harder and uneven surfaces than the training mats in a dojo.

I've taken hard falls on concrete before. It's not fun per se, but it's way better than reacting by posting an arm and having said arm break. Road rash + bruises > pins in arm.

Eventually they teach you to do a fall from a full body height above the ground (~6 feet) which is ~22 km/h fall. Granted, the angles are diffent and forces are somewhat less. But, 20MPH with good gear and technique is generally not that dangerous.

Closest I have personally experienced was a fall off the back of a large horse onto hard packed dirt / loose gravel at a fast trot ~8 - 10mph + 5 vertical feet. But, with the right technique it was basically painless and I did not end up with a bruise or anything.

I doubt that a judo roll will do any good when falling on tarmac at 20mph...

I did a little judo when I was young (only 3 years IIRC, from 6 to 9) and it saved my life many many times, falling from horses, bikes, motorbikes, anything really, at reasonable speeds and with almost no protection.

When falling from something that's moving, you will continue to move when you hit the ground; to limit hurting yourself you need to roll and not skid. The way to do that is to put yourself into a ball, and to do it by instinct (as of course you don't have time to think about it).

It would prevent you from falling with locked wrists and breaking your arms.

You'd still get scraped up and hurt, of course, but falling at 20 mph and not running into a brick wall doesn't have to result in broken bones.

Actually, it does a whole lot. If you have a jacket on, you will avoid getting scraped. And I've fell actually going 20-30 multiple times on the boosted board, as long as you don't fall stiff armed, and have some torque to "roll off" you will be fine. I'm only about 150 pounds though and 5'8", so different body types might not have such a sane strategy.