For outgoing faxes, implement a SAMBA/CUPS (IPP) print server on the Hub. Users just print to it and use standard header info to indicate to what number the fax should be delivered. The hub can rasterize the print job and send the resulting TIFF (that's all faxes are, anyway) to Ooma's servers, which can deliver the fax wherever it's supposed to go.
For incoming faxes, the user just sets up a phone number in a manner similar to adding additional voice numbers. Then they configure their base with a printer to send the fax to (again, using something like SAMBA/CUPS). When somebody sends a fax to that number, the TIFF gets sent to the hub, which prints it on the configured printer.
I know there's some more complex networking involved to make this work (port mapping, etc.) that users may not be comfortable with, and that might not make it worth the trouble. From a software end though, it would be really easy to configure the Hub to do this.