Alright, here's the rub:

I have a bunch of stuff on DO (love it!), and also need to monitor something hosted elsewhere that is part of my app stack. It's external due to RAM, disk, and CPU requirements (dedicated hardware).

I would like to add this external box to the list of machines in the DO console, and see a few basic metrics there.

You have the agent, the cloud console, the alerting feature - all the building blocks are already there.

It shouldn't be too difficult to adapt the DO agent as the box in question is Ubuntu (old, but stable). I don't know the internals of your solution, but I imagine a "phantom droplet" that is not actually connected to DO's infrastructure should do the trick.

Cheers,
Martin