caching vs revalidation in http1.1

There may be cases where an origin server has semi-permanent
resources (that change every few hours or days).  If these change
irregularly, allowing proxies to cache this may be problemmatic (i.e.;
one can not know a proper maxage value)

A second best solution would be to allow proxies to cache, but
insist they perform a conditional get (an If-modified of If-no-match)
before using the cached item.

rom my reading of the ver 3 spec, there's no way of doing this
("must-revalidate" sounds like it should, but it's really a "stale" modifier).

Am I missing something, or is there a notion that well designed caches
will make such an option unnecessary, or ....