I've found a beautiful old stained glass fanlight window that I'd like to install above my living room door on the 1st floor hallway (I live in a 1st/2nd floor walk up maisonette).   It's simply a leaded glass antique stained glass piece.  The hallway in question is technically my "fire escape route".   Is installing that transom compliant, especially if I want to make the property a rental unit in the future?   Or would I need to do some kind of glazing (and likely ruin the piece)?  

Its hard to say other than if its above a fire door then it probably needs to be fire resisting.  If it does then consider having someone install a fire resisting window and then simply mount this inside the frame of that window.