I said earlier that I was going to post the program to extract subkeys from a CHD, and the subkey lists for a few games, however after extracting the subkeys I found important relationships between the subkey bits so I'll have to work on that first.
A subkey consists of 24*4 = 96 bits. There are 65536 subkeys, so the total is 786kB of data.
What I found that one bit of the subkey is constant, while 59 can be derived from the others with a XOR (which is constant for all 65536 addresses, but changes from game to game).
So this hints at a 64-bit global key, while the independent subkey bits drop for now at 96-1-59 = 36 bits.