The folks that run the colo I keep our servers in would beat me to death with a shoe if I did either of these things:

- Mount something in a rack not firmly attached to brackets or a shelf

- Install anything with a battery larger than you'd find in a RAID card

Not to mention all the other ways this is sub-par in terms of airflow, density, serviceability, out-of-band management, etc.

I get the allure of it, but I wouldn't really want my gear anywhere near a bunch of laptops stuck in a cabinet.

It’s not about your battery, it’s the battery in all the other laptops that would have me concerned. Plenty of fire risk.