There is some ambiguity here in that Role Scoper is applying your excerpt suffix to the excert itself (as seen in RSS feeds). But when substituting the excerpt data into post content, the "content" prefix and suffix are still used.
I'm afraid that changing this unilaterally would break existing sites which have come to expect the current behavior. However, think the functionality you have in mind would be more intutive and useful. I've mailed you a modified copy of teaser_rs.php which will get the job done if you first add the following to wp-config.php:
define( 'SCOPER_FORCE_EXCERPT_SUFFIX', true );
With that constant definition in place, the "excerpt" prefix and suffix will be applied to the displayed post content whenever the excerpt, "pre-more content", or "first X characters" are displayed as the teaser. In other words, the "content" prefix and suffix will only be used when the content is fully replaced by a fixed teaser message.
Due to some other current development, this change is not yet in the dev snapshot, but I plan to include it in a Role Scoper 1.1.8 release soon.