Yes, you are correct that the “Maximum lifetime of cache objects” under Page Cache would be used to determine how long arbitrary data is stored. This would only be available to page cache methods that are not disk or disk enhanced (so this would indeed work for memcached servers).
As for being page-specific…although that feature is not presently in w3tc it would be fairly easy to put it in since the Max lifetime is just a variable that is assigned directly to cache processing functions like ->set() for say memcache. I guess the simplest way would be to create it into a usable wordpress filter (or is it an action) that way people could hook into it via there functions.php and by doing a simple check what the existing page is being displayed it would then assign whatever new/different lifetime (in seconds) to use.
Granted this approach would be a bit techy for most people’s taste but at least the feature would be in and available for the more adventurous types.
