In the case of DME and IRS, will the FMC generate some usable combined output during an RF leg?
Yes the IRS provides the, (for want of a better term) raw data position information to the FMC and the DME/DME update refines that position in the FMC, the same as a GPS position would update the FMC position but with GPS it would to a better accuracy (lower ANP).
It is the FMC that "projects or draws" the RF leg for the aircraft to fly. All the DME/DME or GPS does to the FMC position is increase accuracy and thefore the confidence of the FMC solution being where it was designed to be (RNP) this confidence is expressed as ANP (in nautical miles) on the Boeings. If you lose DME/DME or GPS updating the ANP (confidence of the FMC solution) will grow at the IRS drift rate until ANP is greater than the RNP. The IRS/FMC position may actually be bang on, but the
confidence of the position gets worse over time.