Is it correct to call the modifier's value modifiee?

Solution 1:

The following is a summary of the entry for the suffix "-ee" in the OED:

The -ee suffix has two functions and origins. The first originates in Norman French legal terms. The words appear in pairs: verbor -> verbee. (to mortgage -> mortgagor - mortgagee / To appeal -> appellor -> appellee, etc.) However, these are only applicable to persons.

Even nonce words sender - sendee, educator - educatee, relate to persons.

(There are a few words that have caught on that have either their root in a noun (bargee) or no agent noun (devotee.))

The second is used as a diminutive - its origin is "obscure" (I suspect "baby-language"), "bootee", "setee", "goatee", etc., and does not fit your context.

In short - modifyee does not work.