How to allow exceptions to accounts to create tickets via email?

This might work but i can't say for sure as I haven't tested it (but it looks promising):

You could enable that setting (Clients Can Create Tickets Using:   E-mail), then:

1. Create a mailbox in your email environment for your system like:   IncrediblyUsefulSystem@mycompany.com.
2. Create a dummy Client user for your system (or a general one like SystemRequestUser)
3. Create a particular Request Type like:   IncrediblyUsefulSystemIssues
4. Under Setup -> E-Mail -> Incoming Mail Settings   you would create a new entry there and define the Request Type that it uses to be that IncrediblyUsefulSystemIssues one that you made prior.

Now, any emails sent to that email address will create a ticket and assign it to that Request Type (which can be automatically assigned to a Tech Group of your choosing).

How to keep other users from creating tickets via email also?
From what I can see, the "Clients Can Create Tickets Using" setting can be overridden at the Location level.   So if your Clients have Locations you could Edit each Location to set that to only "web interface"

You might need to create a special Location for your dummy client user so that user doesn't get restricted that way.

Ah, with your suggestion I actually found I can do it in exactly the reverse order and only allow it at the location level.  I didn't originally see it until your suggestion got me to looking at it again but in the tooltip it actually tells you that you can use the Location to override the settings.

heh.. i totally didn't think about just enabling it in ONE Location. that's much more straightforward.

Well, I wouldn't have found it without your suggestion; sometimes it takes two sets of eyes to come up with a good solution!  I do appreciate the help!