It's five in the morning and I am reminded again that a failure to communicate is more often a failure on the part of the communicator than the communicated to, so I shall try again.
My premise is that it is possible to have a virtuoso performance of material that does not technically require a virtuoso to play.
I'm not sure if it's singer-songwriters in general you have a beef with or Boo Hewerdine in particular (would this conversation have gone differently if I'd have referenced Molly Tuttle, Chris Tile or Richard Thompson I wonder?) but let's take him and them out of the equation.
Many years ago I was at a piano recital, I don't recall the name of the pianist I'm afraid (I was only there because my dad's parents loaned the spinet that was used for a couple of the pieces) but let's assume, for the sake of argument, that he met your niche definition of virtuoso.

Introducing one of the pieces he said he always felt it was important for a programme to have a couple of bits that were less than 'masterpiece' standard. He felt (if I recall correctly) that it helped provide a bit more contrast and shade to the overall programme.
Assuming that he was a virtuoso, would he be able to produce a virtuoso performance of one of these lesser pieces?
Alternatively, Placido Domingo, as well as all his 'serious' music, has churned out a whole bunch of cheesy pop schmaltz as well. Let's say he headed down to the studio for a recording session for one of these pieces, and he happens to be in a real purple patch at the time, and he gives one of his best renditions. Is it possible that he could give a virtuoso performance on one of these tracks?
I'm reminded of a sporting parallel. Around 30 years ago I remember watching the men's gymnastics at the Olympics. One of the Russian competitors was in the form of his life and had already picked up a couple of gold medals (on his way to a total of five, again if I recall correctly). For his final attempt on the vault he sprinted down the track, hit the springboard, spread his arms and then 'flew' horizontally for a frankly implausible distance before nailing a perfect landing. There was silence and then rapture as people realised they'd just seen something truly exceptional.
Technically, probably any of the other gymnasts could have done something similar. After all, there wasn't a twist, turn, tuck, pike or other move to speak off. But you could tell from their faces that as well as lacking the audacity to try it they wouldn't have had the ability to perform it to that standard.
I'm rambling now, insomnia will do that, but hopefully that has clarified my suggestion that it is possible to have a virtuoso performance of material that would not technically require a virtuoso to play.
P.S. a capo is a very useful device for changing the tonality of a guitar by the way, it's not just for people who only know three chords.
