If the boat
is out of the water
, I've heard you can also disconnect at the iwaki pump discharge, and use compressed air to "back flush" the system from the external discharge thru-hull.
If the boat is in the water... what's your "filter"? A sea strainer with basket? If so you can close the intake thru-hull open that strainer, and pour pink antifireeze in while the pump's running. Some juggling involved to not overflow, and also not lose prime.
Otherwise, you can close the inlet thru-hull, disconnect the inlet hose of the iwaki pump, and still use the iwaki to pump AF through the system.
Another option is to install a Groco flush adapter (aka safety
seacock conversion) in line after the inlet thru-hull. The flush adapter essentiall gives you a built in T in the line, and you can use that (with thru-hull closed) to introduce antifreeze -- or even sediment cleaner, periodically -- into the system.