I do not think there is a built-in way to do that.
One workaround might be to configure an invalid email address in the logins that should not get email on assignment. It would probably be better if it were an email address that causes the messages to be discarded rather than bounced back. Overall not a very clean solution, but it might get you what you need with little effort and few negative side effects.
Otherwise I think you are going to have to script it. Unfortunately scripting is going to suffer from the same problem as your AO profile--the assignment cannot be differentiated from other updates to the alarm. I can picture two approaches:
- The assignment should result in the posting of a message with subject "alarm_assign" to the message bus. You can create a standalone custom probe that subscribes to those messages and then posts a message with subject "EMAIL".
- You could script it completely within the NAS, but I think you would have to dedicate one of the custom fields to tracking whether the email was sent. The script would first check if the field is blank. If not, it does not need to email you. If it is, it would send the email and then update the alarm.
The advantage of the first option is that you are using information that is already available on the message bus, which means you do not need to use any tricks to track state. The disadvantage is that the script has to be a stanalone probe, which is sort of overkill but is not very hard to do.
You will have to decide if it is important enough to make a script worth the effort. I think we can help you piece it together.
(If someone else knows of a very simple way to achieve the desired result, I will feel a bit silly for having proposed these. 
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