Can't you just post your gravatar url as an external avatar?
The whole beauty of gravatars is that it recognizes your email address and sets the avatar automatically. Adding the avatar manually on each site works fine, but this eases the process. The best part is that when you get a new avatar, the change is reflected on all gravatar supporting sites immediately.
I don't know that its absolutely necessary, but it sure would be cool.
Well if you still want it, I have coded a Gravatar plugin which requires no template edits. If you have no avatar, it will check to see if you have a gravatar. If you do, it displays the gravatar. If you already have a set avatar, nothing will happen. It only allows R rated gravatars and lower, though it should be easy enough if you want to change the restrictions.
Post back if you want it, and I'll give you a direct link, though by the time you read this it may already be on MyBB mods.
I think it's a great idea -- too bad it isn't a default setting.
Then I wouldn't have to add my pretty dog avatar manually on every MyBB forum. LOL.