This would be ever so useful. I have just taken responsibilty for the upgrade of a website and I didn't realise that emails were being sent out to all members as I upgraded the site. (As I was creating the pages I wasn't copied in so I was oblivious to what was happening).
I agree it would be useful when making major re-writes downside is remebering when to turn it all back on again.
An alternative is send an email to everyone on registered on the website and tell them you are making changes. Perhaps that will wake people up to it and look at it again. Might also be a chance to get input from people on the changes being made.
I have two email accounts on the system one as admin one not so I see what everyone else sees.....