I used to have a raw water cooled engine, and used to winterize by
pumping antifreeze through it, and closing the cocks. Let the engine
run to temperature so you circulate the antifreeze. Inboards tend to
stay a little warmer since they are closer to the water, which tends to
retain temperature, and protected in some way from the elements. I
would doubt that the first night or two in high 20s will cause it to
crack, but why strain it? I'd plumb a second connection to the
raw-water intake so you can easily and quickly pump some antifreeze
through it, easily enough to do it every time you get back from a trip.
You may also consider a "block heater" type device--I used one in my
car, which did not have an integral one, in -40 degrees. That one was
magnetic, designed for lawn-mower sized equipment, but it worked well
enough: plugged it in, and it attached magnetically to the engine. I
don't know how well it would work against freezing, and I doubt that
you can find one that is appropriate for marine use, so take that for
what you paid for it.

mickey