So, the native colors out of the NES are most accurately expressed in YUV. Those colors can be converted to RGB; I'm not talking about full artifact emulation.
However, on conversion, some of the colors are out of gamut. (So, as a precursor to accurately emulating the emphasis bits without just using a 512-entry table, you need to store
unclipped values in the 64-entry table, and then clip them when it comes time to render)
The emphasis bits effectively
1- Multiply the luminance component
2- Add something to the luminance component
3- Multiply the chrominance
phasor by some scalar
4- Add another phasor to the chrominance phasor.
And even
more of these values are out of gamut once converted to RGB.
Anything that
just multiplies the RGB components by a constant 3-tuple ignores #2 and #4 and simplifies #3 in an inaccurate way.
If you plot the 54-color no-emphasis and unclipped RGB palette in 3-space, and then draw line segments to the corresponding versions of all 54 colors with any number of emphasis bits set (again without clipping), you'll discover that all of these line segments, if extended, would converge at close to the same point.
So: I tentatively think you should be able to emulate the emphasis bits directly in RGB by adding a specific RGB tuple, and then multiplying that sum by another 3-tuple.