Unfortunately (or fortunately for me!) I don't have any bad RAM to test this on.
Maybe the data is right in front of you - the "Bad: ffff7bff" might be the mask you are looking for. We might just be mislead about the statement that memtest86+ is going to present us with an addr:mask result. If this is the case the "good" will be the compliment of the "bad" mask. You only need the "bad" mask for badram:
Umm, is ERR bits the part you need, the "mask". It's all I can see at the moment...
Failing address: 0000b609d50
ERR Bits: 00008000
11111111111111111111101111111111 = Good
11111111111111110111101111111111 = Bad
00000000000000001000000000000000 = ERR bits