Terminal NBD's have a published promulgated range eg. EGKK GY (365)15 nm.
Whereas airways NDB's are considerably higher powered, hence range increases eg. LPPT CP (389) 250 nm.
Although not regularly flown, we find ourselves flying of couple of such approaches a year. (Usually when the ILS is off the air)
Whilst agreeing with the previous comments about GPS equipped A/C (flying an NDB monitored GPS approach) Only a few of our A/C have GPS updating to the FMC
The remainder of the A/C have only IRS with traditional radio updating. Problems occur with the fact that if you are required to fly an NDB approach then by implication there are no other more favourable NAV aids to use. With the lack of more accurate NAV aids the quality of radio updating decreases, and hence errors in the FMC map display can occur (map shift).
My opinion is that on a CAVOK day when you are using the NBD as a airfield approach aid, then monitoring of the LNAV is acceptable. But on any other occasion then the only safe method is to ignor the LNAV and go back to raw data.
And back to the original question, the FMC will preferencially update using DME/DME fixes therefore even when flying towards an airways NDB the FMC will not actually tune the NDB, it will fly to the co-ordinates whilst using accurate radio updating from other surrounding aids.