I took this idea one step further in order to improve on it just a bit....
As you suggested, I added the SMS email address of the handful of people I usually send messages to (at the same time).
But then I went ahead and created myself a new free Gmail account. I updated the forwarding setting on this account so that every message sent to this account will automatically get forwarded to my
number@txt.att.net sms email address.
Then I added this email account to my iPhone.
So now, if I want to text message multiple people, I just create an email (from the new account) and add as many contacts as I want (sms or regular emails). And because of the forwarding feature on the account, when they reply, whether from their phone or email address, the reply will be sent to me as a text message rather than an email message.
Please note that I could have also used Yahoo! Email to do this, but I chose GMail because they don't append an ad at the bottom of every email like Yahoo! does. So the text message goes through cleaner and shorter.