If anybody else stumbles upon this, I ended up just taking off hoses in low points and draining the coolant that way, I seemed to get almost all of it. I had to anyway.
Should be easy enough, but messy, to install a short length of pipe with a quarter turn valve where that plug is to make future coolant changes simple. I added a two foot long piece of hose after the petcock to make draining into a container easier.