Michael, I've used the dye coupler route a lot in the past mainly for hand colouring images, but I've used it to intensify negatives as well.

With negatoves it's rather like using XP1/XP2 film the only difference is you add the coupler to the colour developer rather than it being in the film and you can chose the colour or mix of colours.

It has an adavantage in that a film can be passed through the cycle a few times, dev in a normal B&W developer & fix washn well then bleach in a rehalogenating bleach wasg dev in C41 ith the coupler(s), wash, then repeat the bleach/dev again. This technique was once very common with astro photography for colour films and called looping.

So back to your initial question yes you can add a dye colour, proprtional to the exposure very easily.

Ian