A good reference for these questions is the North Atlantic MNPSA Operations Manual, which can be downloaded at this link:
http://www.nat-pco.org/nat/MNPSA/MNPSA_2008.pdf
Take a look at paragraph 1.3.2, Lateral Navigation. Note that each navigation system must either be an INS, a GPSS, or have input from at least two IRUs or other MNPS equivalent sensor systems. So your scenario with one IRU supplying one FM might not be acceptable.
Note also the requirements for GPS approval, as well as the requirement that each system provide continuous indication of aircraft position relative to track.
Some of these points may help in determining whether your configuration could be approved.