I would say instead of injecting it, what about supplementing its feed with flavoring. As someone said you can get onion smelling eggs if your chickens get into onions. I can confirm that. I was cleaning my garden and threw a bunch of overgrown green onions into my coop, a couple days later I swear I could smell green onion in my scrambled eggs.
So what about feeding them things they will eat normally like green onion, garlic, basil, oregno, tyme, mint leaves, chili peppers. Or you could try adding extracts to their feed like ground cinnamon, pepermint extract, garlic powder, chili powder??
If you were to try injecting them I would try using a diabetic needle, they are small like a 22-24 guage, short and ultra sharp. Instead of stabbing the egg, I would try using it like a drill and twist it to bore a small hole in the shell.
Good luck
Aloha,
Cory