How about doing a scene featuring a "pig" hole style butt plug? For those who've not seen one, these are butt plugs, but with a follow centre allowing the wearer to be fucked through it. To be large enough to fit a cock through means that they're a decent size to start with. They also keep the ass spread open, so that visually it's similar to using a speculum, so ideal for spitting into as well.
They can be a bit tricky to insert - which I actually think would improve the scene - as they tend to be quite soft silicone, and once inserted, the wearer could be fucked with a strap-on.
I've got one of these by Oxballs, called a butt-hole fuck plug (large), which would perfect, but other similar designs are available too.