Not sure Plus Six owners are aware but BMW Z4 production (with the same engine and auto gearbox) ended about three months ago.
However, there is a Final Version called the "M40i Handschalter" this is the same 3 litre B58 engine (produced circa 340-380 bhp) but with a MANUAL gearbox. I believe the Supra had a manual version quite a while ago.
Therefore, in theory the Plus Six Auto to manual is very much a possibilty?