Where is your computer located?
I assume it's located in the den with your MFP.
If you don't have a telephone jack in your den, I assume you're not wired for internet access either, so your computer is connected wirelessly.
The only way I know how to do this is to use a 2nd OBi.
You can use an OBi200 with a wireless dongle or an OBi100 connected to the ethernet port on your computer.
Windows has a feature called Internet Connection sharing (ICS), so the OBi100 will connect to the internet using the WiFi connection on the computer.
If allowed, register the 2nd OBi directly to your provider.
If not, route the call thru the OBi202.
Disclaimer:
I never routed a fax thru a 2nd OBi so I can't guarantee it will work, but I don't see why it wouldn't. I would route the call directly to the OBi202 using your LAN and not the OBiTalk network.