I'm thinking this is my problem. I've never flashed just one img before. Is it much different than normal fxz or do I have to fastboot that sucker?
Well, if you open the *.xml file inside the FXZ directory and take a peek at it, you'll see that it's just a script that uses fastboot to push the appropriate img to the appropriate partitions. I never really realized that until last year (The first time I had a phone that used FXZ files)
Anyway, you could copy that xml and delete all the files except the relevant system.img file from the script and feed that into RSDLite, or you could do it manually with the fastboot from a command line.
I've got a couple versions of the .246 xml on my hard drive for full wipe, for saving data and sd, for just saving sd, and so on. It's pretty easy to just pick and choose the particular level of reset you want that way when you run RSD