Poisson, yes, I think you could use foil as you say, I've been meaning to try it for a long while, though actually I am not such a fan of halation so it hasn't been a priority. Anyway the obvious problem is that you have to take care not to scratch the film with the foil. You could also pre-wash away the AH layer from sheet film. Another thought that I've not had time to try is to make a dupe neg over foil using sheet film with the AH layer washed away. I am guessing that you can add halation to any image that way, as long as the enlarger bulb emits enough IR light. ( And if the IR component is small then a visible blocking filter over the enlarger bulb could probably boost the balance of IR, at the cost of a much longer exposure of course)