I had the same question last fall when I changed my 2GM20F coolant. It has the exact same setup as yours with the hose fitting on the water heater line. I searched but couldn't really find a clear answer for how it's supposed to be used, but ended up buying some hose fittings and trying out exactly what you suggested. It was a bit of a balancing act because I was nervous about over-pressurizing the cooling system with high pressure dock water. I don't think I would've impressed anyone and I probably won't try it again. I also used the air mattress pump trick to blow water out and that was more of a clear winner.
I also remember a suggestion that the hose fitting was for bleeding out air, but I didn't have enough extra hose to raise the fitting above the engine for that, so I just pulled the hose off at the engine freshwater pump and after filling the coolant reservoir, I would fill the water heater hose to the top before quickly inserting it back onto the freshwater pump. That seemed to work well enough despite some spillage. So far so good as I haven't overheated (yet).
I'd still be interested to hear of the proper way to use that water heater fitting before I make another attempt at pushing dock water through all my engine gaskets.
