Ed,
Problem solved (I think ).
Solution: I took extra pains to be sure and overlap my new scenery area generously with the old, and reran FSET and Geoconvert. I'm not sure just how much of an overlap is necessary, but apparently I added more than enough as the scenery is being generated properly now with no low res default tiles showing through the scenery.
This has more than likely been emphasized by Jeff or Rodeo in the forums and I've missed it; hope this helps you and others who may be experiencing the same issue.
That could be the solution!. As you mentioned, difficult to say how much overlapping should be necessary but what's true is that in my case the missing/non-resolution tiles are relatively close to the boundaries of the geoconverted areas, so your conclusion makes sense. I'll try again.
Thanks, Ed