I threw some BMX bars on a mountain bike that previously had old school super long stem and straight bars. Did it more for goofing around and to have a giant BMX bike, but actually put some miles on it cruising around and it's not bad at all.

Threadless BMX stems are standard 1-1/8" steerer tube clamp, so no problem there. BMX stems and bars are generally 22.2mm, or 7/8ths clamp diameter. Although I think some 25.4mm stuff is floating around now. You will likely need a new 22.2mm stem to match whatever bars you get, if only because you need some serious BMX stem clamping action to keep 10" tall bars from tipping forward or back.

Also they are available is almost any rise from 5" to 11" or so. Dans comp is the gold standard of online BMX shops: https://www.danscomp.com/

Just to add to the solid info you've already received, keep in mind that BMX stems are short --- 60mm is about as long as they get, but 48 or 52 is more common -- so factor in the effect this will have on your reach.

As far as handlebar rise goes, you might be surprised at how high even a 6" cruiser bar will place your hands. If you're looking for lowboys, Fairdale's MX bar is 4", and Surly's Sunrise bar is 3.25". Also keep in mind that BMX stems can be top-load or front-load, and your choice will slightly influence the effective bar height.

I know origin8 makes a BMX style stem that fits riser bars so you could get that stem and I would suggest BMX racing bars because they're lighter and shorter. Danscomp.com sells them. Also if you're using it in the winter I would suggest lock-on grips. Odi and lizard skins make great ones.

Too goofy for me, I tried to put I think 6" BMX bars onto my FGFS SE bike. It felt pretty crappy even with a bike that's semi made for tall bars 4" is where I would draw the line I think. I settled on bars that were 60mm which I think is about 2.2" somewhere around there. And about 24" wide.

I've been contemplating putting some bmx bars on an old steel mtb, I worry that they will slip forward with any type of impact.

BMXers and dirt jumpers have been putting bars and stems through every kind of impact you can imagine for years and years. With the proper clamp fit and torque, BMX bars are as stable as any other.

You should be fine, I wreaked havoc on a beach cruiser with riser bars and a 1 bolt clamp off and on for like a year. I never slipped the bars, I would just check them here and there.

Speaking of the topic anyone know of any good manufacturers of BMX bars that make them in North 'Merica (you know Canadia or Mexi Co. or the Yooknighted Shash)?