comment_approved custom value

I’d hazard a guess that it isn’t possible — at least not in this exact way — because of that default: return false; part of the code you reference: a custom value of the comment status will hit that and the function will stop immediately.

However, have you thought about using a comment metavalue to store the same data? You could:

1. Set the value: Set a custom meta on the offending comment, say pluginname_comment_hide, to true (I like removing negatives from values — e.g. hide is clearer than nodisplay).
2. Don’t display the comment: This is trickier but I believe you could adjust the meta_query value in the parse_comment_query filter in WP_Comment_Query to remove the offending comments. (This might affect that back-end as well, so you’d want to do it conditionally.) You could also use the callbacks args on wp_list_comments() but you’d end up with comment counts that are off.

I don’t know if that’s too complex for an answer — let me know if more information would be helpful.