I don't think spray on would provide enough padding for full time stalling. If they have an outdoor area where they spend most their time it might be OK. Lots and lots of bedding can also compensate for lack of padding but that gets expensive and a pain to clean.
About 8 years ago we purchased equifoam mats made by Dow Chemical company. They are lightweight and can be cut to fit. We put them in our two horse bumperpull mainly to help with road vibration. We're still using the same mats! They clean easily, provide traction and soften the vibration from the road. We trailer at least once a week, sometimes for hours. Our horses come off rested, and they hop on again with absolutely no problems. No slipping or sliding. I'm not sure where you'd get this now, but you could probably google it. We LOVE our trailer pads and so does everyone who sees them and walks on them. It's a lot like a padded mat you'd put somewhere like a shop where you stand a lot.
OOPS...I just realized you're talking stall mats and I'm talking trailer mats. Sorry.
It is possible to get lighter mats than what are traditionally used in stalls. Equimats North America - The Mats With A Revolutionary Design
Equi-Mats are very light, I have 3 that I haul to shows with me to put in my show stall. I'm a 120lb girl and I can easily move them around, although I do use a wheelbarrow for distances. I've had mine for almost 2 years. My horse's borium studs on his shoes did tear them up a bit, so they are not as durable as the heavy black mats. But they work and I know a few barns that have Equi-Mat installed and they are happy with them.
Joe, the equifoam mats we use were actually developed for padding electronic equipment when it was shipped. Then some of the people at Dow started using it as floor mats when they were doing trade show --- and one of those people had horses! Eureka! They come in long, rectangular panels that you can lift and carry just like a foam panel and yet cut with a knife. And they are TOUGH. A lot of rodeo people use them because their horses are trailered all over. It really reduces leg fatigue. We trailer ours to trail ride and our horses are trailered every single week in a two horse bumper pull. We've never had any leg issues. Ever. These pads/mats really are amazing!!