It looks like this just isn't possible. At least for this phone on the Straight Talk network anyway. I have tried everything. I tried using an xml editor, exposed, manual edits to the build.prop.....No go. I got it somehow to flash for a second with a different carrier name but it always went right back to "HOME". So unless someone else can figure this out I and other in my similar situation are stuck with this generic branding. We could always flash another Rom. But in my case, I kinda like the stock Rom.
EDIT: I talked to 3 different people at Straight Talk and they are what everyone says they are. More useless than tits on a bull!..They are telling me they cannot send me a new card even though I proved it's the sim card. I put my card in my wifes phone, you know, the one that does say AT&T? and her phone also said "HOME" with my sim card in it. So that clearly shows it's something in the sim card itself. But according to those morons my phone is working so there's nothing they can do about it. ST has pretty good service for the price. But their customer service is completely missing.