Awesome I was so paranoid they said solar posted it and I searched all over the forum and couldn't find it. I was scars I accidentally broke a rule
The only time this would be against the rules - is if they tried renaming it and passing it off, trying to sell it to you as one of the promo items - when in actuality it was just a normal horse... (even though they would be lored, and you could tell difference from the custom names on the eggs)
You could always talk to Aikar and ask if he would change the Naming Tags to charge 5 xp Lvl. just like the anvil does.. But no, it is not against the rules to rename stuff using the anvil, it is part of the core client to allow renaming of items via the anvils. The name tags were only added as a little special item that does the same thing without charging those 5 xp Lvls that could be used for repairs or enchantments.. :3