I think what prevents is the general rule - you basically cannot place a district on any tile with a feature. Unless this feature is removable, e.g. Marsh, forest, jungle. Floodplains are not removable, so you cannot place a district there at all. Egypt can place districts there but that's an exception done via modifier. Not sure if it can be reused for other purposes.
Edit. There's an effect ADJUST_VALID_FEATURES_DISTRICTS, maybe it would do the trick.