The other song to “get the fretless treatment” is Remembrance. You may remember this from the Musicians for Haiti project last year.
In addition to re-tracking the bassline with the fretless I did a lot of general “clean up” of the project (moving inserts to group channels when possible, etc.) and relevelled the appropriate tracks. To fatten it up, I dup’d the strumming tracks in the middle, delayed by a few milliseconds, and panned the resulting pairs hard left / right. Finally, I unmuted a fourth picking guitar track that I originally recorded, dup’d / delayed / panned that track too.
The result is a much more open sound yet one that doesn’t sacrifice on aural quality.