Dear Stuart,
You haven't reported back to me. There are two bits in this register, one bit is the presence of FMC card and the other is power good. What the software does is to make sure the FMC board is present assuming power to be good, therefor only checking for bit 1.
The software source code has a comment saying PG_M2C (defined by Vita standard) is not available on LPC therefor not checked.
The actual value you would expect is 0b11 on an HPC carrier. If the GPIO switch is not set, you would read 0b10 on a HPC carrier.
Let me know if you need more information or I can close this topic.
Best Regards,
Arnaud