I haven't done this (yet), but a few sailors on my dock have installed a diverter Y-valve, similar to what sd mentioned. I was inspecting an installation just this past weekend on a friend's boat - simple to do.
He actually uses his to divert flow from the seawater pickup to an attached hose leading to a bucket filled with freash water. If sitting at dock for a while, he flushes seawater from his raw water section, replacing with fresh. This increases the life of the engine, heat exchanger and pencil zinc, which he claims to change only once every 2 years.
Of course, using a 50/50 anti-freeze & water mix in the bucket would be just as easy to do.