Is it possible to access contribution tokens in hook_civicrm_tokenValues?
I think I can self-answer this after exploring various methods to create end of the year thank-you letters for recurring/multiple contributions - the donation tokens are not passed in the $values array, and are not otherwise easily accessible from within the hook function.  

In theory it may be possible to access an array of contribution data passed in the Smarty variables, but it was way easier to simply make a custom token that does it's own queries to create a small table showing all donations made in the previous calendar year (including donations made by a spouse)...