In quantities you could 'eat' naturally, without injecting it in by force? Yeah, I think that would be entirely safe. (Though you wouldn't want to breathe exclusively neon.)

But, almost all of any gasses you manage to get into your stomach will be expelled as burps. Very little will make it through your entire digestive system and out the other end. If you want neon gas coming out of your ass (I presume to use in conjunction with some electrodes and a power supply to produce actual plasma), you're going to need ... alternate means of ingesting the neon. The simplest and least invasive way would be to stick a tube up your ass and pump neon gas into it through that tube. For your own safety, ensure that the maximum pressure on that gas line is very low. You do not want a ruptured colon. That would hurt. Plus, the neon gas would escape.

Also, even if successful, I wouldn't recommend it. Plasma is, you know, hot. And assholes don't like being very hot.