search result excerpt/snippet – use AIOSEO description?

Sure, but it has nothing to do with Relevanssi.

Your theme decides what is displayed in the search results template. You can modify your theme so that it displays the AIOSEO meta description instead of the Relevanssi excerpt.

AIOSEO keeps the meta description in a custom field _aioseop_description so all you need to do in the search results template is to display that:

echo get_post_meta( $post->ID, '_aioseop_description', true );

Got it – thanks so much!