Sort posts after filtering them through multiple taxonomies

Question

I'm using scribu's Query Multiple Taxonomies to filter posts through multiple taxonomies (which works great!).

However, I'd like to allow people to perform further sorting on the results based on custom fields which have numeric values (eg. rate_per__private_session or rate_per_group) by clicking on a link.

I thought that something like the following would be all there was to it but clearly it is not.

<a href="<?php query_posts($query_string . '&meta_key=rate_per_private_session'); ?>">Sort by Rate per Private Session</a> | 
<a href="<?php query_posts($query_string . '&meta_key=rate_per_group'); ?>">Sort by Rate per Group</a>

How do I do this?