Update:
no brick!
I plugged in a USB-powered chill mat. both to keep the thing cooler as temp regulation was non-existent, and to drain the battery quicker. Power went off after 2 hours, without even the charge-indicator light flashing beforehand.
I plugged the A/C power back in. Machine stayed dark; good. Hit the power switch and it came up as usual. Booted Linux ok. Rebooted to OpenBSD and only thing was it had to check the filesystems first and mark 'em clean.
Seems to be ok.
Whewww. Hope I won't fall into that one again too soon.