Totally a great idea M! I have dyed lots of stuff in my washing machine using the sachets you get from Spotlight with good results. They come out very evenly. You need to add salt to the machine. You also need to do a empty wash afterwards to get the dye out the machine - never had any trouble with others loads ending up dyed.

Cotton dyes the best - you might find that embroidery bits or embellishments or stitching won't dye or will come out lighter.
i used to use the powders you use in a bucket, but the sachets in the machine do a much better job. Some of my threads don't dye the same as the material due to the material being cotton, and the threads being polyester.

Also, if you are doing alot of dying, check out buying dyes from the US (same ones as the spotlight ones) if you buy a few there are much cheaper to get from OS even after postage costs.