It's on the private board, reposting it here:

include "vga.h" in render_scalers.cpp, and change to the line
#define PMAKE(_VAL) Scaler_PaletteLut.b32[_VAL & vga.dac.pel_mask]
in render_templates.h

As stated above, it is NOT advisable to really use it as is.

maybe the more correct way would be to add the pelmask at writetime and not a rendertime.
but for 2 games I'm not that happy to add it.

You can't do this at write time as the c4 vga modes directly
access the video memory.
But it can be merged into the palette handling, yet i'm not
sure if there's an easy way to do this when the host system
uses 8bit graphics.