Most people do halochrome toning on prints, but you can use it on a traditionally-developed (i.e. not pyro, not chromogenic) b&w neg or positive too. I did it, and I wish I could show you something, but I am certain that it really won't scan well.
Halochrome, if you buy it, is kinda pricey. You wind up using a lot of it just trying to get a few toned prints. For negs it is much less expensive- you can treat film with very small volumes.
Incidentally, I understand that there is a toning book with the halochrome formula in it, which I haven't seen but have read about. I think it is "The Photographer's Toning Book" by Tim Rudman. Google and see. Anyway I am thinking that you could take the halochrome recipe and adapt it to whatever you want. I have had some stray thoughts on this and just haven't had time to follow through.