As long as you put it on the threads only, it should not hinder ignition, if you got it inside the chamber area of the plug then there might be a problem.
Use it on the threads, once together, clean barrel inside and out with hot soap-water then oil well.
Use the anti-seize sparingly, don't cake it on and if you're still worried about getting it in the breech, try this, stuff the breech with a small, scrap cloth.
Assemble the breech and then use a patch puller or worm at the end of your ram rod to snag the rag from the breech, then pull the rag out through the muzzle than clean with hot water and soap, then oil.