I'm pretty sure that doing the standard rev 6 upgrade to 1mb will just stop the trapdoor ram from working. You can probably hack the motherboard and add extra logic that allows both sets of memory to be used, but I haven't heard of anyone ever doing that. So you'll be the first to figure it out.
I did basically that, except I piggy backed the extra 512K on the chips on the motherboard because I didn't have a trapdoor expansion.
