I don't know if it's possible to do this but here is a suggestion:

A blank image (of any color you chose) that would fill the whole screen, and which you can adjust its transparency.

e.g so the dashboard



with this on top



at 70% transparency gives this:



with mieru:





EDIT: here's with a black image overlayed at 30% transparency


(this can make it dimmer than n900's lowest brightness setting while still readable)


It's basically how Screen Filter for android works (and why it doesn't require root)