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Does anyone use a rootstock to specifically dwarf larger species of cacti? I know we usually graft to increase growth, but it would be interesting to know if some of the really large species could be kept pint-size by doing this.

Wow that's something I've never thought of doing. I imagine some stock that's tiny and doesn't grow large would do the trick, but you also want it to be resistant and hardy for a long-lasting plant. But there's another way and that's basically doing what bonsai ppl do to trees, keep it in a small pot and it will stay small. You'll have to prune roots or it will get root-bound. My opuntias that are in small pots grow pads that are like 1/5th the size of the ones that grow in big pots.

I've never heard of anyone doing that, but it seems possible. Maybe a small species of Opuntia, like fragilis?

You could always graft it on a Blossfeldia! Sorry not being serious, however I can't see you would really gain anything trying to dwarf Cerei. Better to take new cuttings by topping periodically when they outgrow your conditions and re-propagate them?

Any Western film fan would want a bonsaiguaro, though, wouldn't they?

Grafting is for weak sensible roots.
If root good, there's bonsaing - small pot, poor soil, less water.
As a stock Eriocereus jusbertii cas survive in worst conditions.