Actually even this (easter egg from game with seemingly contradictionary settings) can be done without breaking an immersion. Just quick example (one of the less elegant ways to do it): local fool with oddly shaped piece of wood claiming it is great magical weapon capable of killing anyone at distance; actually just pickable “odd piece of wood” would be sufficient plasma gun reference (Or not really pickable, if you try it the character just claims it looks to rotten to be of any use.)
It doesn’t even have to be gun reference. Off hand comment about someone living among stars and lot of other things I didn’t thought of.