Yes, there is a VERY well known problem with the R800at - it does not come with fastboot at all. Learned this the hard way after messing with the R800x quite a bit, and then buying an R800at off eBay. The only way to enable it is with a service box containing the right SETool software. The only person I know who has this is @Alejandrissimo but I'm not sure if they're able to work on those phones anymore. I spent several days (not an exaggeration) trying to find other ways of doing it, but wasn't successful.
That being said, it is still possible to use some custom ROMs and kernels on that model. I have the Lupus v12 kernel (if I remember correctly) with AuroraPlay Lite on mine, and I recommend that setup. If you want a custom kernel, you will need one in the FTF file format, which you can flash through FlashTool like regular FTF firmware files.
Otherwise, the phone is pretty much identical to the other versions. You'll just have to do a bit more research on a ROM before installing it, to make sure it's compatible.
Alright. Good to know, and thank you for the information. I now know how to proceed. Dang. I got this phone years ago from att, did not think they would continue being such ***** but they are. I only wanted to upgrade because I want to run printoid to control octoprint and thought it would be cool to use the included gamepad to move the axis and maybe have a little fun free form drawing something, if that were possible.
I know I require android 4.0+. Off to search through the roms. Have a good one and thanks for the heads up.