MC-1000 doesn't provide the external ROM that MC6847 may use to get the characters bytes when it is in "External Alphanumerics" mode (AG=0, AS=0, INTEXT=1). Currently the emulation doesn't provide a cfg.get_char_rom at VIDEO_START, and thus it works as if a byte 0x00 is read. In the actual machine, though, MC6847 puts the character code byte into the wires trying to reach the external character ROM, and then reads the wires again for the pattern... but it gets the very same byte that it put there, i.e., the ASCII code of the character.
I think you could add something like...
UINT8 mc1000_get_char_rom(running_machine *machine, UINT8 ch,int line)
Then @ VIDEO_START:
cfg.get_char_rom = mc1000_get_char_rom;
The result is not usefull at all, as you can see by accessing this MC-1000 Java emulator
but that's how it works, so here you have it.