Maybe I have been doing it wrong, but I don't think I have ever seen an RN number on any shirt with a custom label that isn't put on it by the original manufacturer. Same goes for country of origin unless it is a made in the US blank and being added as a selling point. Usually that seems to be simply left off. Fabric content, care instructions and size are standard, with logo and website almost always included as well.
I imagine the reason for the RN and country of origin not being included is on a lot of runs I have done with custom tags at least some of the shirts have different countries of origin and different RN numbers, meaning you would need new sets of tags for those. Add to that the fact that the RN is often included on a separate tag down at the base of the shirt instead of by the neck, and it becomes even more of a PITA. I'm staring at two Anvil black shirts that were next to each other in a stack of 12 from my supplier and they have different RN numbers and different countries of origin, but they are the exact same shirt...
Like I said, maybe I and lots of other people are not doing it by the book, but thats just what I have seen.