Actually, there are NO ethics involved on this one. Just edit it until you get the image where you want it, just like you would in a darkroom. The neg is simply your starting point. If you have the image where you like it, you're done, no matter how you did it. No one has to like it but you, or agree w/ how you've made it that way. The only "rules" are, if you exhibit a print in a gallery (vs posting an online image), it should be a "photograph". Meaning, if you've done anything non photographic to it, such as painting or marking an area w/ a brush or ink, or gluing something to the surface, then it's a mixed media piece, not a photograph, and should be clearly described as such.
It's like that w/ all art, and photography is an art form. The minute you lay a brush to an etching or lithograph, or use any sort of alternative media with, or to, a painting, it's a mixed media piece, no matter what it "looks" like. It's more about proper categories than rules. You have more latitude w/ a negative vs a finished print, as the neg is just an intermediate step in the process. I would imagine that if you marked up, or otherwise altered a negative, and then printed it conventionally, you would still have a photograph. These rules are necessary in order to prevent buyers from buying a pig in a poke. No one wants to purchase one thing, and then find out it's something altogether different once they get home. The art world is full of people trying to fudge the boundaries on all of this.