I don't have any experience with memory cards outside of my digital camera etc but can't you use it basically as a "raw" memory simply by NOT formatting as FAT or whatever. I mean the protocol is likely (haven't looked it up) different than a simple SPI flash IC and without a "proper" file system you won't be able to read/write it from the PC but you can't really do that (easily in this context) with a "raw" SPI flash IC either so what's the difference?

/Henrik.