I would like to set a site wide DATABYTES variable, but allow certain users to bypass this...can this be done?

Yes...this is possible.

You can set the DATABYTES environment variable to control that. If you know who they are by IP address, you can use tcprules to change that variable depending on who's connecting. If you're using SMTP AUTH, you'll need to find an SMTP AUTH solution that lets you set environment variables depending on who connects.