This seems awfully like the sort of info that will be queried in different manners for different cards, i.e - I'd expect it to be hardware-dependant. Should this be the case, the magic is happening inside the device-driver.
How patient and good are you at reverse-engineering for a single device?
Hard-disks don't need drivers, so they can use an agreed interface, which you access with DeviceIoControl. I have little hope (okay, basically 0 - but also, not much more knowledge!) that this will prove to be anything but a painful, tedious process - one that may still have little to no hope of being fruitful.
As an exercise, try downloading the datasheets for each of the NRF24L01 and the a7105 2.4GHz chipsets. I worked with them a few years back, and from memory, they have similar, but quite different methods to retrieve the RSSI value. You'll be looking at the same situation with the the NICs, except you'll have very little idea (if any) of how to drive the correct lines high/lo as needed and to listen for the response, since you can't connect directly to the 2.4 GHz chipset, and instead have to go through the mcu on the board. (Hence the reverse-engineering suggestion)
a7105:
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^]
NRF24L01:
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^]
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