Thanks for the replies on this. I am gradually improving at transposing on the fly. Definitely easier in some common places and shapes (capo 5 with G shapes is a common one I use to end up in the key of C).

With regards to BIAB, I found the easiest way for myself to write out a song is to just put the chords in based on the shapes I am using, then just shift the key up however many frets I'm putting the capo and Bob's your uncle!