""There would still have to be an official organization, otherwise anybody could just up and call himself an imam."
Yep. That would be the case. Though not everyone would have to agree, or listen to him. It is more of a bottom-up thing than a top-down thing."
Not that much different from Protestant Christianity, really. Anyone can start a church and declare him/herself a minister. You can just start preaching. If you can attract a following, then you have a viable Christian church. There are so many of them around here that were started just that way.
You are correct with regard to SOME Protestant denominations, but not all. Some (Episcopalian, Methodist) have fairly rigid hierarchys. Some (Church of Christ - NOT United Church of Christ) have no ordination, no structure, and have individual congregations run entirely by elders. Under inquest's construct, some of these would be "religions" for 1st Amendment purposes and some would not.
I would note, however, that Moore's view does not follow that line of thought. He thinks "religion" meant Christian denominations. I've tried to find my source for that - a piece he wrote - online so I could link it. I haven't yet.