While this is not exactly on topic, I'm curious. Why do you think that the Namecoin scheme does not work and that no other coins would adhere to that scheme? I'm at least aware of three other chains (Namecoin, Huntercoin and Xaya) that all follow that scheme and at least one pool (F2pool) that mines more than one together.
Check it yourself from the algorithm given in the wiki. Also the wiki states this. If merkle_size is a power of 2, when there is a slot collision of slots for some chain ids, then the collision will persist no matter what merkle_nonce you use. I can sent you a code function that checks this by scanning but it's clear from the equations this is true.
So I don't know how f2pool is doing it, it will work if it's merging 3 chains, but not for merging 4.