> A little more forward looking:
>
> In such situations I often get this thought: if the variable was
> replaced with a function accepting zero arguments, then we could use
> `add-function' and `remove-function' to control the return value of the
> "binding".
>
> I know the advice mechanism has the reputation of only being suitable
> for end user customization/hacks, but in cases like this one we could
> make the modification of the value more explicit and controllable.
> Dealing with interferences would be forced to be taken into account more
> directly , and we would get some useful mechanisms like priorities or
> looking at the context out of the box.
>
> We could also invent some even better mechanism, maybe.  But in this
> case binding variables doesn't look like an optimal approach at least.

This mechanism looks like a variable watcher enabled by `add-variable-watcher'.
So you could add a watcher that conditionally controls variable modifications.