Allow hooking a custom python function to overwritten values

[uberspot]

It would be very useful to be able to execute custom python code whenever a value is overwritten during reclass.

Some use cases would be:

1. Debugging: figure out if a value was overwritten somewhere when it shouldn't have
2. Analytics: see which values are overwritten and which aren't. (This might be useful for cleaning up duplicate overwrites of values in certain cases).

Any idea if this is possible already or easy to implement?