Ok, did more reading up, and had a good look at this
page, particularly the part about Device IDs.
I've checked up the respective datasheets for the different capacity chips, and the 1 GiB chip (K9K8G08OUM), has an id of 0xd3, which doesn't seem to be supported. There goes my dreams of a 2GiB player...
If you want to put the 512 MiB chip in your player, I think there's an 80% chance it would work fine.
I also had a read through the datasheets concerning piggybacking, and I'm absolutely positive that it would work just fine - all these memory chips have the "chip enable" pin, which allows for piggybacking, as long as the firmware supports it.
According the the rockbox link above, the author says that the firmware should allow up to 4 chips, which is good news for piggybacking.
As for piggybacking chips of different capacities, I believe it will not work.