I recall using this methodology a number of times. The usual implementation is to file a feature request ticket in the issue tracking application, add a comment explaining why we aren't doing it, then close the issue as 'WONTFIX' or 'INVALID' depending on how things are set up.

I agree it's really powerful to have the list and to consciously and actively say "no" with the same ceremony that you consciously and actively say "yes" to features.