This is an awesome idea. Currently Drake only supports timestamped and forced evaluations, but it would be great to have an evaluation abstraction where you could provide your own implementation of whether a target's changed and/or whether a target is to be considered fresher/younger than another target. Timestamped would compare modification times, forced would return true, and it could be extended indefinitely.

If you're serious about it, please submit a feature request (https://github.com/Factual/drake/issues), and describe more specifically what you would like to be able to do in your case.

Thank you for a great thought.

Artem.