This project started out as an experiment but has worked out very well.
When they tried to mount my ITP Mudlite tires on the standard 7" rim using tubes, the tube exploded blowing the side of the tire out and sending the mechanic to the hospital for stitches to his hand. We decided to go tubeless but finding the right valve stem took some time. ( plus I had to order a new tire... ouch. )
There was some question in my mind if it would work but after inspecting the wheels and finding no cracks, we installed the tubeless valve stem and the tires mounted like they should have and held air. I have been running this way for a couple of years now with no problem.
I have not tried these with a set of standard Mule tires. When I removed my original tires, they were tight and very hard to break the seal. I believe the original tires could be run tubeless.
The ones I used can be found at Gimplers