It's more a question as to whether there is any worth in doing it.I don't see why not, maybe best to use the same rule as pitching onto yeast cakes though, like go from a paler, less full on beer upwards.
I'm all about doing the best beer I can with the tools/ingredients I have (who here isn't? ). If throwing a little slurry into the boil will give me healthier yeast in the ferment, then yippee. If it's not going to do anything, then I won't bother.
I think it's more about the flavour of the slurry - the slurry will have some hop flavour, unfermentables, etc, that will impart flavour, even if only a little. It doesn't take much chocolate malt to impart a definite flavour in a beerI wouldn't think the same issue applies here because the old yeast isn't producing any flavours it's killed and used as a nutrient rather than processing fermentables.