It is unusual for this type of rug to have this type of backing or any other for that matter, save the back of the face fibre.
They are usually quite dark as you have said, primarily green or red but obviously a variety of colours.
These will wet clean very well. If you clean them the way you have suggested you will get very poor results and maybe end up with a "stiff" textile that will need brushing.
You know I will say this but the best way to clean them is with saturation.
If you are going to use a hwe unit then do this.
First of course remove as much dry soil as you can from both sides.
Then, face up, hose it down to absolute saturation point.
Turn it over and then complete the saturation.
Then with or without a cleaning solution designed purely for this type of cleaning, brush like mad the entire rug. If you have a rotary with a softish brush then use this.
Then hose it down and squeeze the moisture out. A wand will work but dont have it connected to any machine, just the wand.
After the first 2 or 3 times of doing this (if using a cleaning solution) use just water then just keep going maybe 5 or 6 times. Each time after you saturate, you brush, you saturate you squeeze. You can do this an unlimited amount of time till the water runs clean.
Turn over again so the face fibre is up and do it all again.
Don't forget it can take many times of doing this until the water runs clean. Then and only then will the rug be clean.
When you know the water is running clean you can use the wand as an extractor. Just keep going over it again and again till no more moisture is coming out this way.
Now just dry it and you "should" be fine. As long as you know how to dry it properly. Sometimes if not all, this is where the problems can arise.
These rugs dont (usually) bleed so they are very good rugs to practice on. They are also very short pile (also usually) so will dry quickly which under normal circumstances is what we strive for.
Hope this helps. If you really want to experiment with these rugs you have then make a "pool" or "tank" and do the same thing as above. You would just need to empty the pool between each clean. If they arent that soiled then you shouldnt need the tank.
Best, Dave.