Maybe it makes sense to apply your acquired knowledge on Pegasus mailboxes to Mail::Box, so you would be able to make use of all of the nice features of Mail::Box, and the rest of us would be able to use Pegasus mailboxes with Mail::Box as well.
Yes, it makes sense to write such an extension: I welcome all contributions. I am not familiar with Pegasus mail, but will support anyone who tries to implement it.
MailBox uses OODoc, which means that it is simpler to write consistent documentation. If you decide to start coding based on an existing folder implementation, please ask me for a "raw" pm file (which contains both the OODoc POD and the code).