That's what I was afraid of. That's a hefty requirement for something of little gain. To that I would say do not bother mapping anything to 40-4f:0000-ffff and allow it to remain unused as it is now. The expanded meg is already accessible via 10-1f:8000-ffff + 90-9f:8000-ffff and it would be fast access at that for the 90-9f region vs. slow for everywhere else. I don't see any reason one would need to map it to 40-4f, nor would want to access it there. This way, the original game will still function perfectly fine and the cart modification is actually reduced to adding the extra address line and you're done. Nothing more than a single wire and two solder points.
For record I spoke to byuu about this. I glossed over the fact that only half of the expanded meg would be accessible via 10-1f:8000-ffff + 90-9f:8000-ffff. Doh! So, 40-4f would be the only place the entire second meg would be accessible if needed.
My proposal is still a valid expansion and would indeed be better suited for a cart modification, but he said he absolutely requires the full meg. So, that's that. I would like to see an easier original cart adaptation (ideally one that even fits with no expansion at all), but at least it will make it more difficult for bootleggers if it's harder. I do hate those bootleggers.
It also appears Tom now has a version with his script inserted to play with, so I will rest easy now. Carry on gentlemen.