OK here is an update I seem to have got the thing working with SPI on USCA0 of the MSP430G2553. I just modified pins_energia.h and one file of the existing SPI library (usci_spi.cpp). By working I mean I can hook up my SD card module to the A0 channel and with this code I am successfully logging to the SD card using the (unmodified) pfatfs library.
pins_energia.h I added a preprocessor flag #define SPI_USE_CHANNEL_A0 1 to remap the SPI pins constants
// wnorcott 5/18/2015
// Uncomment the following ifdef to use USCI channel A0 instead
// of B0 for SPI. That means you can use hardware SPI and I2C
// simultaneously on the MSP430G2553 but you give up hardware
// serial capability. The G2553 has two SPI channels A0 and B0.
//#define SPI_USE_CHANNEL_A0 1
#define SPI_USE_CHANNEL_A0 1
static const uint8_t SS = 7; // P1.5
static const uint8_t SCK = 6; // P1.4 UCA0CLK
static const uint8_t MOSI = 4; // P1.2 UCA0SIMO
static const uint8_t MISO = 3; // P1.1 UCA0SOMI
static const uint8_t SS = 8; /* P2.0 */
static const uint8_t SCK = 7; /* P1.5 */
static const uint8_t MOSI = 15; /* P1.7 */
static const uint8_t MISO = 14; /* P1.6 */
changed SPI library module utility/usci_spi.cpp to test the new flag and variant the code to use channel A0:
UCA0CTL1 = UCSWRST | UCSSEL_2; // Put USCI in reset mode, source USCI clock from SMCLK
UCA0CTL0 = SPI_MODE_0 | UCMSB | UCSYNC | UCMST; // Use SPI MODE 0 - CPOL=0 CPHA=0
UCA0BR0 = SPI_CLOCK_DIV() & 0xFF; // set initial speed to 4MHz
UCA0BR1 = (SPI_CLOCK_DIV() >> 8 ) & 0xFF;
UCA0CTL1 &= ~UCSWRST; // release USCI for operation
... etc. in other words changed all the register references under the #ifdef to use USCI channel A0 hardware registers instead of channel B0. The methods I had to monkey with under the #ifdef were: spi_initialize, spi_disable, spi_send, spi_setdivisor, spi_set_bitorder, and spi_set_datamode. It would be nicer if it were possible (and maybe it is) to add the #define SPI_USE_CHANNEL_A0 1 in my own sketch or at least pass it as a -D flag to the compiler when I build the sketch, instead of having to define it in pins_energia.h Thanks again for your help.