It's been done in PNG, because the locals where ripping up the power cables to the NAVAIDS. Least expensive solution was to replace all NAVAIDS with a GPS waypoint and authorise "GPS may be used in-lieu of NDB, using NDB waypoint"
Pretty sure there was NOTAM issued by the PNG authorities about 2 years ago, not sure of the current status.
I also know there were issues in Timor a few years ago with NDB's going out of commission and therefore being no approach available...similar solution was implemented by certain ADF assets in the area.
I think the concern might be that by doing this it becomes more a psuedo NDB/DME approach, rather than a pure NDB approach with timed legs or allowances for different cats of aircraft. Also think there might be a gps nav performance issue....but can't be 100% on that one.
Still, I reckon its got to be more accurate than tracking a steam driven gauge over mountainous terrain or coastal areas....I think if implemented properly it could work.