Ear muffs are very easy to use for both inboards and outboards. So should inboards be flushed? Have a look at the price of heat exchangers which should answer the question, they are not cheap.
My Pacific will be spending large amounts of time on a trailer, however I would like to run the engine up without having to launch her each time. Because she draws water though a sea cock rather than the stern drive ear muffs can't be used. Here is a pic of the modified top of the sea cock with some 3/4 inch marine plumbing so an ordinary hose can be attached. With the main sea cock closed and the red valve opended water flows into the engine heat exchanger and out through the exhaust. Cost of the bits £11 and half an hour in a machine shop.