I finally found the full list of short-codes that can be used for the page title & meta description in the Yoast SEO plugin. They are in /wp-content/plugins/wordpress-seo/inc/wpseo-functions.php. It starts around the middle, with $replacements = array( …
In the current version, they are:
%%date%%
%%searchphrase%%
%%page%%
%%pagetotal%%
%%pagenumber%%
%%caption%%
%%category%%
%%excerpt%%
%%excerpt_only%%
%%focuskw%%
%%id%%
%%modified%%
%%name%%
%%tag%%
%%title%%
%%userid%%
%%category_description%%
%%tag_description%%
%%term_description%%
%%term_title%%
