I can now answer my own question. It is not a 'nice' solution but it works and may help others.
The following would seem to be the logical way to change to using the Alt CS on this LCD shield but this fails.
bool MTDS::begin() {
// return begin(pinMtdsSelStd, frqMtdsSpiDefault);
return begin(pinMtdsSelAlt, frqMtdsSpiDefault); // My change for alternate pin selection
}
In MtdsHal.h there is code to define the two IO pins used for CS, pinMtdsSelStd and pinMtdsSelAlt, for various types of hardware.
Following all these #'define if elses, just force t