There will have to be a couple of differences between the way rejected subscriptions and rejected messages are handled, one of them somewhat major, one of them minor.
The big one (which hit me in the middle of the night) is that if you implement my suggestion to add a composition box for the (always optional) message for a rejected subscription a la rejected message, you would need to explicitly add a "yes" or "no" (at some point in the process - not sure where the best place would be for this) to "Send a rejection message?" That's because rejecting a message *requires* a notice to the member, even if it's just "Your message was not approved" (with no message added for the reason); whereas with a rejected subscription, groups currently have the option to send no notice at all. Currently, the options are to (a) send the "active" notice every time or (b) send no notice at all. You would need to retain (b). So the code would not be completely parallel to rejected messages, because another yes-no would need to be added. But still not a big deal. Just not sure where the question should go. (And, of course, you could have a "your subscription was not approved" notice go out with no added message, simply by not filling in the message box.)
The minor differences may involve logging, since the rejected pending member is not a member of the group. I'm not sure how this is handled, since I've actually seen an original rejection logged under an email address after the email address is later approved. So my guess is that if the email address has a confirmed account at the time they're rejected, the action still gets logged under their email address somehow. Of course, the rejection action would still be logged under the group and moderator activity, with the text of the notification message included in the log, as with rejected messages.
Messages are the sole opinion of the author, especially the fishy ones.