The coolant vent plug part arrived and I was able to remove the hard line and install the plug.
- removed two 10mm bolts that fastened the hard line to the engine block; one on top and one next to the hole
- pushed the black plastic ring in, wiggled and rotated the hard line, hoped nothing would break, and eventually it moved and slid out; nothing broke, yay!
- pushed the ($20!) plug into place
Hopefully we'll get the engine installed tomorrow or the following weekend, and all will work as normal.