I have done it that way. I used enough antifreeze in the fresh water tank to fill the water heater, then drained the water heater right away after. It can be done. Getting to your water heater by-pass is much, much , easier way. You say it's not accessible? Can't you find a kid to squeeze in there and get to it? Got any kids, or grandkids? A neighborhood kid you could hire?
Last year I Drained everything. Put the plug back into the water heater and blew out all the lines with a portable air tank. It took a couple of portable tanks of air until I was satisfied. Then I put a gallon of pink in the tank and ran the pump to make sure the pump would not freeze. I keep my TT in storage at home so I kept a electric heater inside of it with the temp set to about 60 degrees. I made it through winter this way on -20f below nights.
My water heater does not even have a by-pass. I wish it did.