Came across this site that has blades for skateboards
www.extremeblade.com/en.html

Could be used on an atb for sure. The best would be to get the unsharpened version and do some hand grinding - or on a disc sander with carborundum disc to a 45 degree angle.
Sharpening so that it is hollow ground like a hockey skate won't give enough bite for the sideways pull of a kite. For the wacky skateboarding application it would work. A friend had tried sharpened hockey blades mounted on a skateboard with a hand held sail and there were side slip issues, those issues would be worse powered up with a kite...

You figure those would be okay on cracky, bubbly lake ice? Wouldn't a longer blade be better?

If it was too crackly or bubbly I'd be on skis for sure.
The attached picture is an ice sailing board that uses mountain board trucks with hockey blades mounted. My wife isn't so good on skates and the windsurfing rig is super easy for her to handle.

Smooth ice... warm chinook winds will often melt the snow and if it isn't windy when it refreezes we get days like this...

The blades at www.extremeblades.com do seem a little short, but they were the first commercially available blades for a board that I've seen. On the right surface they would work just fine.

Actually looking at the blade holder of the extreme blade it would be possible to mount a longer blade if one had material to do so. I'm not sure how durable these holders would be with a lot of side load...

We have tested the iceblades on a flexboardz last year in Canada (see here)
http://www.dailymotion.com/relevance/search/flexboardz/video...
Blades designed for skateboards are a bit short but they are now developing larger blades for mountainboards (and Flexboardz).
Sideloads (and grip) could be an issue with a kite/sail on a mtb...but the tiltable wheels(blade) should greatly improve it.