I have Toshiba satellite 2805 s503 that uses an Intel i815 which from i under stand uses normal CL2 and CL3 ram. so i don't totally buy the proprietary ram thing specially when thing were standardized then as they are now why would toshiba waste money making thing proprietary the cost benefit from buying only from them still wouldn't be worth it even back in P3 days.
Here is what I think really happened the Satellite 2805 series was market is mid range laptop and had anything more than 256mb ram locked out so that would have to buy a higher end
satellite or laptop.
It could truly be incompatible ram as there is on board ram which would be enough to facilitate the error message it could be as simple as too high density i will have to do some research on the chip set via Wikipedia.
EDIT wiki says the max the chipset supports is 512 so maybe that is why it won't take 512 chipsets but, still wonder what would happen if that safe guard is removed heck i wonder if you could turn off the onboard ram and is the entire 512mb chip rather than a max of 384 using the combined onboard. OR maybe older in 256mb onboard ram but that would be trick because A: the onboard ram uses the same resistors and caps for its various on boar rams sizes and just uses different ram chips, or B: the most likely the 64 and 128mb on boar ram setups are different using different caps and resistors possibly making it in possible to switch out the on board ram making it more likely to mod the bios to use 512 mb sd100/133.