The reason you don't use mdadm is because it keeps track of bad blocks by default, although that can be disabled, but when using the debian installer to set up arrays it sets up the config file in /tmp then blows away any changes made when dropping out of the installer to modify the temp conf on return to the installer, making it impossible to set up a md array that doesn't track bad blocks when using a serial console (although if you aren't, you can open up another terminal to do it) for the install?

That's precisely the reason you don't use mdadm?