“The goal: On each post/page to be able to specify a custom title, description, and set of keywords.“
Title
The normal functionality in WordPress is not enough here? Then Dmitriy’s reply points to a possible solution for this.
Description
My Head META Description plugin is worth a look:
http://guff.szub.net/2005/09/01/head-meta-description/
It provides a description based on an excerpt of the content, the excerpt text itself, or a custom field (‘description’) value for every post or Page. But there are other plugins out there which handle description tags:
http://codex.wordpress.org/Plugins/Meta
Keywords
There are several plugins for this as well (check same link above).