Thanks riha, also thanks for the polite correction, you are right, buttermilk has basically mesophilic not thermophilic as I mistyped in my post above.
This means that the bacteria is really dead and thus no need to sit at room temperature overnight as nothing good will happen, just give a chance for bad airborne bacteria to take hold. To keep the meso bacteria in buttemilk alive, you'd have to come way way down in temp to 105F/41C or lower to keep it alive and get a more cheesy taste.
The barely boiling step is really to pasteurize (or re-pasteurize if store bought) the milk. Normally after pasteurization, you drop temp quickly, maybe you could do that and then add the buttermilk . . .