I'm pretty sure the Allies matrix doesn't count Daemons and Astra Militarum as Battle Brothers, so they can't get in each others' vehicles. I suppose you could put 20 bloodletters in a CSM Spartan from Forge World, but... that's about the closest you can get, I think.
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!