I have seen this myself and never have liked it.
But the answer to your question is - in my opinion.
it is not a code problem.
The owner (i.e. governing body - can risk assess anything)
Review NFPA 99 2012 Edition Section #1.3.4.2.
(sorry i did not cut and paste, but sometime it's better to look things up and read the text ourselfs).
On the part concerning no nitrous oxide piped to the location were the wagd inlet is located. Again review NFPA 99 2012 Edition Section #5.1.5.16.
Another fun read. Its simple, we have to install a wagd inlet anywhere N20 or halogenated anesthetic gas are INTENDED to be administered.