Well, actually, there are serveral slightly incompatible formats, and they all are essentially append-only formats. A variant is used by Netscape's Communicator and its derivates (Thunderbird, Seamonkey). It has a separate index file containing the file offsets of the e-mails in the mbox file and some other data. "Compacting" the mailbox just rewrites the entire mbox file, omitting all e-mails flagged as deleted. (See http://www.jwz.org/doc/mailsum.html.) Mail servers using the mbox format probably do something similar. I remember one old server at my university that kept a special first message in the mbox files, invisible via POP3 and IMAP. Parts of this message were constantly overwritten to keep track of changes to the mbox file. I think that some of my old linux machines behaved the same until I've switched to imapdir several years ago.
So, perhaps the only useful ideas from mbox are a second file containing a list of file offsets into the main file, and a special (fixed size) directory record at the beginning of the main file.
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)