If you are able to drive a 5/8” steel rebar to put the magnebed on, it will work quickly and efficiently in rip rap. Once of the two times that I have dealt with riprap I was able to pull up a few rocks and get a stake down for the magnebed. If you can not drive a stake, then the magnebed will require a custom “base rock”. What I did was remove the stake-socket base and attach the magnetic top to the edge of a brick with tapcons. I then “packed” riprap around the brick to securely wedge the brick in place. This worked, but as with other methods like the sod or bag/pantleg filled with sand, it requires some extra preparation and effort and care when setting. Which goes with the territory, thats why they called you, you are the pro with all the tricks who is working for a living…. Im right there with you. Faster is better, but I do what it takes.
Next time I encounter riprap I have a new plan. I am going to weld up a three legged support of heavy 5/8” rerod like an almost flat tripod. My idea is to remove a few of the rocks in a triangle, put the three legged base down and get it set where it needs to be, then put rocks back on top of the legs to secure them. I expect it will be easier and all around better than the edgewise brick method, and the legs should only need to be around 14 inches long to get it nice and solid.
If you dont have other uses for the magnebed like very soft mud or steep banks very often, your best bet for a one-time deal will probably be the sod or bag/pantleg filled with sand.