@raybills,
I'm curious about what advantage you see in writing the SD card without a FAT.
Since you say you have already interfaced to the SD (with a FAT?) I assume you are aware of the FileLogger and DevicePrint libs discussed in the Exhibition section? Both seem to be a reliable and "low cost" way of writing the card, and being able to read the card from a "PC" seems like a nice perk for most applications.
As I said, I'm just curious and also want to make sure you are aware of these other methods. Rolling your own seems like a lot of work. 