An "in-the-oven" meat thermometer may be used as an instant read thermometer. . .
Have done this for a couple of years now. Works fine, especially when you cannot find the meat thermometer when you are placing the very expensive prime rib roast into the oven and you are VERY reluctant to cook with the "minutes-per-pound" method (because it always seems to be overdone when you do so) and THEN you find the meat thermometer after the roast has been in the oven for an hour.
Works for us.