Can't change UEFI/kernel boot video resolution to something supported by head unit

[Mel Pilgrim]

I have 11.2 installed on a pair of Supermicro X11SSH-LN4F mainboards 
(C246 chipset, Intel Xeon E3-1200 v6 series).  They use UEFI-only boot 
mode for the NVME system disks.

The system boots just fine, but when the FreeBSD UEFI loader runs, it 
switches the console video resolution to 1600x1200, which isn't 
supported by the 1280x1024 monitor of the rack's head unit, and is also 
so large as to be cumbersome when using the BMC remote console.

I know I can set a video mode in the loader.conf, but I get an empty 
list when I run `vidcontrol -i mode`, `kldload vesa` gives me the 
notorious "error 19" message, and randomly trying resolutions doesn't work.

How do I switch the loader and kernel to a lower resolution?