The PJW Page Excerpts plugin lets you specify an excerpt field for pages. It stores the value in the post_excerpt and works just like it does on pages. Could your Meta Keywords and Description plugin take that into consideration? All you would need to do is change line 109 in meta-keywords-description.php to this:
if ((is_single() || is_page()) && have_posts()) {
Making this change would not affect users without the PJW Page Excerpts plugin because their excerpts would always be blank and the default excerpt would always be displayed for pages. But users with the PJW Page Excerpts plugin would get this nice extra feature.
http://wordpress.org/extend/plugins/wp-meta-keywords-meta-description/