But, either way, I'm assuming I can't do this (making up syntax):

import Data.FiniteMap using MyPrelude as Prelude

So that every place in FiniteMap that Prelude functions are called, MyPrelude functions are called instead? Then I could simply redefine just and nothing as return and mzero.

Not possible with GHC, I'm afraid. At least, not at the moment. We would have to retain a strong separation between Data.FiniteMap and Prelude, such as you would do if the Prelude were dynamically linked, and GHC doesn't do that. Indeed, doing so would probably have a significant impact on performance, because the compiler wouldn't be able to take advantage of any knowledge about the Prelude except for the types of functions when compiling Data.FiniteMap. Cheers, Simon