Has it been mentioned that there is a recitative and aria with lute obbligato.in the St. Matthew. It is Nos. 65-6 ("Komm süsses Kreuz"). The earliest version of the Passion calls for lute, but later versions use viola da gamba, a particularly unfortunate choice because with all the leaps the obbligato is very awkward on that instrument. But it nicely fits the natural gap between the thumb and fingers of the lutenist. There is a complete facsimile of the manuscript of the earliest version in the Neue Bach Ausgabe.
Andre Burguart (I can't spell his name) was working on a solution that makes the obbligato in the St Johna's easier to play. Is that the version that Andi Schlegel is printing in Fronimo? If so, it would be worth the while for thiose inyterested to check it out. He was going to publish hix "solution" but I haven't seen it yet. AJN.