Hi Alicia,
As I understand it, the Allow Null Values property of a field in a geodatabase feature class or table is part of the underlying schema and therefore cannot be changes on the fly. What kind of editing will your users be doing where you want to implement this behavior? I have heard that Survey123 for ArcGIS can implement this kind of custom behavior in a configurable (non-code) way.
If it's for a mobile application but Survey123 doesn't work, you could probably program this custom behavior using one of several ArcGIS Runtime SDKs.
Finally, if your editing needs to happen in desktop, I think the best way to do that would still be to implement a custom Geodatabase Class Extension using ArcObjects.
I can definitely see how it would be nice to have this ability to implement conditional schema properties in the geodatabase. I guess since there's so many layers that sit on top of the geodatabase (mxds, services, web maps, applications, etc) Esri is nudging customers in the direction of implementing this custom behavior at the application level.
Good luck - let us know what direction you end up going.
Micah