It would be interesting to see if it would boot if if the data was split into four 1MB sections each and rearranged on the board.
Looking at MAME it seems that the data is loaded completely differently between original and XP boards:
Original boards (non-XP) actually load the data in U84 and U71 first, and then continue to U83 and U70 for the the next part of the data, with everything being interleaved across the four chips. The data is stored in two bytes between chips e.g. the first two bytes are read from U84, then the first two bytes of U71, then U83, then U70, and then back to U84 for the second lot of two bytes etc.
XP boards only need U83 and U70 as they have twice the capacity but are interleaved between only the two chips, with U84 and U71 being reserved for the 5th and 6th game chips (4MB each).
For it to theoretically work you would have to dump the data of U3 and U2, de-interleave it, and then interleave it a different way across four 1MB lots so it could load on a non-XP board.
After all that I can see why the System EPROM board really is needed!