I was told (and have read) that tarp, bivy, and hammock camping are not allowed at Philmont. I'm glad to read here that this is not the case in real-world experience.
One of my former scouts (who was a Philmont guide/ranger last summer) told me their main concern is being "fully enclosed", and they really prefer floors/net inners to keep the creepy crawlies at bay. He said something about the bears being "trained" through experience to know the food isn't in things that look like tents.
So basically, if your tarp can form a mid, you're good to go.
Personally, I'm still having trouble letting go of a fully enclosed tent, but I'm very sensitive/allergic to bug bites.