I wasn't around at the very beginning of the mojoPortal project, but if I recall correctly, I think I saw that when it started email address was the only way to sign in. So if the core of the system is premised on email address being unique, that might be a tall order to change (database structures, etc.).
I know you said you proposed workarounds. Did you think of using a "+" in the email address to add uniqueness that's ignored by the email system, like sharedemail+Joe@gmail.com, sharedemail+Jamie@gmail.com? Those would both end up being routed to the email@example.com box, but should be seen as different in mojoPortal. If this sounds like it would work, you should definitely test it first to make sure it works before proposing it to the client.
I hope that helps,