405 is an interesting standard to generate at 12MHz, but that's probably a different thread altogether
405 Line NTSC is certainly possible.
I've just completed, though so far not tested, a new version of firmware (VHDL) that, along with a revised PROM layout, allows a total of 18 (yes, 18!) Teletext pages to be produced by the same hardware, without losing any of the previous functionality. This has taken many attempts and incarnations of the code, as it really didn't want to fit into the available 64 macrocells! Fingers crossed it works out....