Not sure I understand your question - you want to stop people downloading from your Google Docs site on PCs outside your network, or on PCs you don't have Host DLP installed on?
The requirement is for Internal Laptops traveling and inside the LAN, with HDLP Installed. All my sales task force are working with Google Docs, but i need to protect that info by downloading files to a usb or something else.
Since the data is in google docs, and is probably untagged via the conversion (as you mention), how are you going to stop it leaking out via home machines etc?
I'm not sure anyone here has the expertise to help you though - you might want to contact your McAfee reseller and get some prof services. I would think that if the transfer to USB happens via a HDLP protected machine, you could block based on content, but of course all your tags I expect will evaporate in the conversion to native gDocs format.