Per MPSSE command codes version 1.5, neither ACBUS8 nor ACBUS9 is accessible from MPSSE mode.
Per FT232H data sheet, the ACBUS8 and ACBUS9 pins default to tri-state inputs, but can be configured to support
ACBUS BitBang by first connecting an EEPROM to the FT232H and using the FT_Prog utility (http://www.ftdichip.com/Support/Utilities.htm) to set IO Controls for C8 and C9 to I/O Mode. This needs to be done once, to configure the non-volatile EEPROM memory.
Next, at runtime, the FT232H needs to be configured for CBUS Bit Bang Mode by calling FT_SetBitmode with ucMode = 0x20. This is similar to what you're already doing to enable MPSSE mode -- obviously by entering CBUS Bit Bang Mode you are also exiting MPSSE mode. So that's inconvenient.
It's unclear to me which bit physically maps to the FT232H's ACBUS8 and ACBUS9, this could depend on how the EEPROM is configured. There's a document that describes an earlier version of ACBUS BitBang mode for FT232R, but that chip doesn't have the same ACBUS configuration and so isn't directly useful.
Just a suggestion, but if you don't need to distinguish between the individual power-good lines, you might consider using a wired-OR to develop a system-power-good signal that you can read through MPSSE mode.
FT232H data sheet:
MPSSE command codes (AN 108):
Bitbang mode for FT232R - ACBUS BitBang description: