How to modify an existing meta description?

Looks like a syntax error.

Change this:

return( $title );

to this:

return $title;

EDIT

Sorry; mis-read your question.

I believe the question is entirely Theme-dependent. The description normally is output in the template via bloginfo( 'description' ). The bloginfo() function uses get_bloginfo(), which uses a simple switch, and for the description parameter, the function simply returns get_option( 'blogdescription' ).

So, I’m not seeing any filters being applied anywhere in that process.

Have you taken a look at what any of the myriad SEO Plugins do with respect to the description meta tag?

EDIT 2

Any ideas?

Short of writing a core patch, to add a wp_description() wrapper function, complete with apply_filters() call, around get_bloginfo( 'description' )? Not really…