I remember that proposal, and it would've definitely supported strictified operations. On Sat, 21 May 2011, Edward Z. Yang wrote:

...

As it stands, we have strict functions for certain operations, such as insertWith. But this current selection is somewhat arbitrary. For illustrative purposes, here are other functions that ought to be strictified in IntMap, because there is no easy way for a user to implement them.

Is it possible to solve this with the proposed Location interface?

"Library proposal: add a Location interface for element-wise operations on Data.Map" http://www.haskell.org/pipermail/libraries/2011-January/015593.html