The date went really well overall. I did in fact go with a Schoeps MS pair, Mk4 and Mk8, into and dematrixed by a SD 788T. I also recorded the mics "as is" on two other tracks, which was good since when I played the 50/50 back in the studio, there was too much (splattery) room which was quickly fixed with an adjustable MS decoder plugin in my DAW. Lots of stereo audience even after I turned down the side channel a bit.
The forte piano is pretty mono sourcy; just a little piano on a wide stage that sounded better corrected.
The instrument is an antique (duh!) and the bench was a little squeaky but the musician was very aware and so not a problem. The HVAC in the church was another issue but was brilliantly cured with Izotope RX, my first experience with this for me.
All in all, a nice evening's result.
Thanks to all for your input.