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That’s why going to an open-source BMC is important.

Do you know of any open source-compatible uninterruptible power supplies? I specifically chose this model because APC has (had?) a reputation for being bulletproof.

(My last UPS was also closed source. It liked to beep at odd hours, and sporadically turned itself off. At least the APC doesn't do that, I guess.)

Also, at the end of the day, it's just an overpriced sealed car battery. I wasn't looking for a hobby project, and if I was, this would be low on the list.

It's more than a sealed car battery: it's also a transformer, some MOS FETS, a charger, and switcher (basically a tiny version of an Automatic Transfer Switch that a datacenter would have). https://www.youtube.com/watch?v=Fj7e3WGUKO8 has a good teardown / explanation of different types of UPSes.

Eric S. Raymond was working on one, with the help of some EE friends, but his blog went 404 some time ago. You might find some mention of it on archive.org, and perhaps contact information for one of the other developers.

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