Hello Ionized,knowing you have a basic understanding,I'll keep my response brief and invite you to ask detailed questions if you want to. Most any of those little pumps will switch 120 volts,,,,,,,,BUT,,,,,,,,it is dangerous. If and when the pump fails,and sooner or later it WILL FAIL,you have 120 volts in contact with water. You can find what you need in commercial hvac system parts and ground source heat pump parts. On the cheap,one can be made up from spare or salvaged parts. A source of 12-18-24 volts controlled by pump switch,then 24v to a remote relay which would send 120 volts to your 120 volt device.

Thx, I suspected that homebrew might be a good solution. I've got a transformer sitting in a bin in the garage, left. By the previous owner so I guess I am 15% on the way

How about this 24V to 120V relay switch,

That looks like a neat package. Too bad that the power supply for the trigger is not built in. I see that they have on an add-on module, but a wall wart from the drawer would probably work.

It looks like I'd need a normally-closed switch on the pump. I'm guessing that is how they are typically made since it would be more fault-proof that way.

Yes,single pole normally closed will work but can be improved with dpdt so that transformer doesn't pull power 24/7,or could be wired to shut down on any chosen event.

Yep, make something up with the transformer, some wire and insulated prongs.