A trivet probably would not make much difference for a roast in a 350 oven with plenty of liquid in the bottom.
However, if the heat source was only from the bottom, ie, stovetop or fire/hearth, then it could help keep liquid circulating under the roast and inhibit burning. Anything liquid would help keep temperature down with a trivet, but would not be as effective without. Burning can occur in areas of restricted liquid circulation, like between a big chunk a meat and the bottom of a DO. As what little liquid is present becomes steam and the pressure moves it out from under the roast, only the meat is left. Continued steam now coming off the overheated meat keeps whatever liquid is around the roast from getting under it, and the meat browns then blackens. There is nothing circulating to let heat escape other than thermal diffusion, which slows with reduced water presence.
Trivets may not PREVENT all burning if the heat source is too moderately too high, but it may be less.
Trivets probably do not do much to limit burning if the heat source is too intense, as whatever is in the liquid burns to the bottom of the DO.
My 2 cents. Don't spend it all in one place.
