It will be a song played only with Nigel on the UK shows? Or a song played without SAX also?
My guess is that Nigel will ad sax parts to songs that can be played with or without sax (what it is, why aye man, romeo and juliet, i use to could, so far away etc) and probably it would be a song or two that probably will be in the waiting until Nigel arrives to be played.
Yes, I think R&J is a banker. Perhaps that is the song they are playing in the picture - we seem to have forgot, in midst of all the speculation, that Guy said they are running through an old tune (or words to that effect). Remember if Nig is playing in R&J then Richard will not have his solo at the end as this will be taken by the sax, hence the different guitar... 
even when R&J was played with sax, the 2nd guitarist always played electric guitar, mostly for chords in the chorus, and during the 2nd verse, and some lines at the end (not particulary Richard's licks)
but you are right, a song can always been rearranged, so it's just speculation
