In a new Eberron campaign I'm starting soon, I've tightened the availability of dragonmarks slightly, by not allowing any aberrant dragonmarks and requiring every character with a dragonmark to be the correct race. One of my players, however, has asked if a character who used to be a human, e.g. a shade or revenant, could have a human dragonmark. Is this possible? Is it actually dealt with somewhere in the eberron guides or is it just my call?