Say I install a driver (FC HBA) that immediately blue screens the server (Windows 2003). If I reboot and select Last Known Good, will that enable me to get to the login screen? I know it loads the previous registry.
generally yes, last known good configuration is legit if the system crashes prior to the windows logon splash screen. if it's able to bypass the logon splash screen and logon then last known good configuration would no longer be valid and safe mode would be the next steps.
That may work. If that fails, you could also try Safe Mode (F8). Removing the HBA may let you get in also. If all fails, you could try a pre-boot environment (WinPE or BartPE) to access the filesystem and make necessary changes to drivers or configuration
I've had last known good config to be finicky. I find if I really hose something up, I try safe mode and try a system restore to the nearest point before things went wonky.