archive.worldofdragon.org

If I remember right, the original daughterboard has a jumper that switches between the standard and inverse video character sets.

What you can do programmatically is change between the 6847’s INTernal character set (the standard Dragon one) or the EXTernal one (the one in the daughterboard).

A bit of disassembly of the ROMs and then some searching on here...

CLS 9 switches the G0 value in text mode and determines whether character set 0 or 1 from the ROM is used. It stores details of the mode setting in bit 4 in zero page $77.
This isn't switching INT/EXT character set.

Sixxie and co already nailed it in Xroar since 2013 it seems! The latest MAME version I have, not so much...