I understand that, that was a very minute side idea to the primary idea, of allowing an exception to your implemented redirect and again Subscribe2 was just an example.
So forget branding and forget Subscibe2 completely. Those are just distractions to what I'm talking about now. Please re-read over the replies to see where the discussion went.
The question is now, how exactly are you setting the redirect to block users from wp-admin? I didn't find a physical .htaccess, perhaps you're implementing a virtual one? That way users could try and manually add an exception.
The follow up to that is would you consider actually adding a field where we can specify specific admin URLs to allow for our users. The idea being yes we generally want to block users from the admin, but on occasion may need to allow them access to specific admin URLs without allowing them access to the whole admin.
I totally understand if the answer is no, that's not something you're interested in adding, just need to make sure you understand what I'm saying first.