Erik Bosgraaf is an all-time top-notch hero, and has a mutant tongue that's capable of things far beyond the speed at which any normal person's nerves can carry an action potential. What he does is not biologically possible. But it's beautiful.
The thing I love about Van Eyck is that his music works at so many levels. Bosgraaf brings out heavenly beauty, but it still sounds pretty good played by a lesser mortal. Many of the tunes, in their basic form, are appropriate to someone who's only been playing a matter of weeks (Wat Zalmen op den Avond doen is a good example), and yet we can keep revisiting them time and time again as we progress, and more and more variations become accessible. It really is universal music. The full set of the Lust Hof is a lifetime's enjoyment for any recorder player.
Yes, I play using descant fingering on the treble. My playing on a genuine descant isn't great, and I have neighbours - also my descant is the one I had at school, and it had a little accident with my child and ended up glued together, so it's not what you'd call a perfectly-toned instrument!
My explanation of the horrible difficulty of playing a recorder on an unexpected clef, even when it's a clef we can easily read for a different instrument (e.g. when a keyboard player turns to recorder and tries to use bass clef) is that recorder is very different to a piano: you can't look at the lowest C and treat it as interchangeable with the next C up, in the same way as you can on a piano. With any keyboard if you want to transpose an octave you can just move your bottom 8" to the left or right and everything continues exactly as normal - there need not be a mental link between a particular C on the stave and a particular C on the keyboard, and all C's on the keyboard look identical. Recorder is different: two different notes with the same letter on the same stave have to be treated totally differently. So if we move to a new stave altogether, we may still know that the note is C, but we haven't a clue what to do about it, because we don't automatically know which C...
I might be completely wrong! It's certainly much harder to switch around. That's also why I struggle with one of the things Anacrusis mentioned, dealing with situations where the music moves outside the range of the recorder. If the best option seems to be to shift up or down an octave, I find it really hard to do. The old players must have been amazing; it's not at all rare to find situations in 18th C editions where the tune suddenly changes from treble clef to soprano clef(?) for a handful of bars. Scary; I couldn't do it.