I just cut a hole in the top of the bag outside and stuck the garden hose in it until it ran clear
Sounds like a pretty simple way to do this.
I tried it once with some lava rock. I punched a LOT of small holes in the bottom half of the bag and ran the hose in through the top, but it did not clean the lava rock very well. I had to dump small amounts into a bucket and stir it a lot with the hose running. I only needed a little bit of rock, so a couple of buckets with only about 1 gallon of rock at a time worked well. That was some really dusty rock, though!
I have usually rinsed substrate in the tank because I can stir it around a lot and keep the dust stirred up as it is being siphoned out.