Yea, that is a board made by BCM.
You can find the specification that you need on the formfactors.org web site. Here is a link to the ATX specification: http://www.formfactors.org/developer/specs/atx2_1.pdf. To save you time, here is the description for the PWR_OK signal:
PWR_OK is a power good signal and should be asserted high by the power supply to indicate that the +5VDC and +3.3VDC outputs are above the undervoltage thresholds of the power supply. When this signal is asserted high, there should be sufficient energy stored by the converter to guarantee continuous power operation within specification. Conversely, when the output voltages fall below the undervoltage threshold, or when mains power has been removed for a time sufficiently long so that power supply operation is no longer guaranteed, PWR_OK should be de-asserted to a low state. The recommended electrical and timing characteristics of the PWR_OK signal are provided in the ATX/ATX12V Power Supply Design Guide.
Motherboards should be designed so the signal timings recommended in the ATX/ATX12V Power Supply Design Guide are used. Using these recommendations will help drive the industry to an acceptable standard.
(This second spec is also available on the formfactors.org site)
The answers to your questions are (1) See above, (2) No, (3) No, and (4) No.
Now, there is a separate Power-Good signal on the motherboard that is generated by power sequencing circuitry and feeds into the chipset (PCH Component). You can look at the various chipset specifications to find out if there is any protection for HDDs via this signal but I rather doubt it. When you abruptly remove power from the system, any operations in progress are going to be stopped at whatever point they are at. A checkpointed file system design might be able to recover from this and avoid corruption. Your best bet is to avoid this happening. How? Provide a UPS capability and do a orderly shutdown when A/C power is removed.
Hope this answers it,