[Plugin: Regenerate Thumbnails] Call for specific attachment ID?

Yes, look at how the ajax_process_image() function in my plugin works.

Basically it’s this though, although you should check for errors along the way (see function):

// Get path
$fullsizepath = get_attached_file( $image_ID );

// Generate meta data (this makes the thumbnail images)
$metadata = wp_generate_attachment_metadata( $image_ID, $fullsizepath );

// Save the meta data so WP knows the thumbs exist
wp_update_attachment_metadata( $image_ID, $metadata );