tal cache: directive could support an age variable

I would like the tal cache directive to support an age variable that I can calculate instead of hard code into the template.

The milestone is a good example. We see a lot of timeouts related to large lists of bugs (many of which are private). In it's active state, a release manager may be changing the bugs and specs several times an hour. We want a low cache time for active milestones. When the milestone is inactive (complete or released), we do not expect the information to change, so we want a high cache time.

I think I want to do something like:
    <tal:bugtasks
      define="cache_age view/bugtask_cache_age"
      content="cache:private cache_age">
Where the view will return "10 minute" or "3 hour"