Why can’t you create a group mailbox with unlimited recipients? Just call it GMG-All users or whatever you like.
I think what OakenMan means is a Public Group because by definition and function a mailBOX is a single recipient regardless of how many people have rights to it.
You could create a resource of a shared mailbox (this can only be done through powershell mind you) and do with it just what you have described. Make the recipients unlimited as you suggest and assign full mailbox rights (this can be done from the exchange console) only to those who need to send out more than 100 emails. We have departments who share mailboxes in this fashion and it works quite well. I suggest the mailbox rights via exchange console rather than delegates as delegation causes many unintended issues I won’t even begin to enumerate here, just trust me on that one.
Another suggestion I have really depends on who the recipients are going to be. If say for instance you want to send an informational email to your entire organization and that will include more than 100 recipients, then create a public distribution group with everyone in the organization incuded and then only allow only a specific list of users to send to that group. Exchange 2007 counts a distribution group as one recipient no matter how many people are in the group. Restricting who can send to it prevents rogue chain mail letters from being sent to the entire organization with multiple replies “to all” to stop sending it, Yeah we’ve had our share of those too before restricting who could send to that group.
If the recipients will be different every time or recipients outside the organization then you can create a different Public Distribution List for each group you need one for people inside the organization, but if they are going to be outside recipients then the shared mailbox granting specific people full rights sounds like that would be the best solution.