Troubles booting Pi2 from USB using bootcode.bin method

I've got an early Pi2B (not plus) that has been booting reliably
from a USB2 disk connected via a USB3 hub using just bootcode.bin
and timeout on the DOS partition of the microSD card.

A recently-configured USB3 disk was set up using 
FreeBSD-13.0-RELEASE-arm-armv7-GENERICSD.img 
but if it's plugged into the hub the Pi2 won't do
anything at all on powerup. It's as if the Pi3 disk
somehow locks up the Pi2 very early, before any
output is presented on the serial console. 

If the USB3 disk is connected after boot is complete
the disk seems to work normally, so I don't think
anything is wrong with the disk. The same USB3 disk is
in use on a pair of Pi4s without difficulty. I've updated 
the bootcode.bin file on the DOS partition of the microSSD
card to the present version offered at
https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#special-bootcode-bin-only-boot-mode
but that made no difference.

This probably isn't a FreeBSD issue at all, but if anybody
has seen this or can offer suggestions on what to try it'd
be much appreciated.

Thanks for reading,

bob prohaska