I once buried the bottom of a Euphorbia ingens cutting 3 feet into the soil, and the tree grew like gangbusters. But that would be an extreme exception.
The reason why I recommend not burying the plant in question is that its ID is not settled in my mind. E. trigona is ridiculously easy to start from a cutting, some of the other similar Euphorbias much less so. Rather than test the limits of each plant, I prefer to give them the benefit of the doubt. Bad things will happen when you bury too much stem of a Euphorbia cutting, as I have discovered several times.
On the question of callus, the distinction in my mind is not between cactus and Euphorbia, but based entirely on exposed surface area. Some cuttings you can make with a tiny nub of a cut stem, because of the shape of the plant, and those you can pretty much stick right into the soil. As the cross sectional area increases you have to provide more and more time. With a wide stem (like on a wide columnar cactus or Euphorbia) it can go up to a couple of weeks. You can rush the process by powdering the exposed stem but the easiest thing is to just park the cutting somewhere until it's ready. The alternative is wait overnight and plant the cutting in soil, but be sure not to water for the rest of the required time it takes to heal.
