Raspberry Pi POE+ hat overlay

I have a pi4 with the official POE+ hat which I want to run FreeBSD on. One
thing I noticed is that the fan doesn't run in FreeBSD so I decided to try
and write a little driver for it. First step seems to be to load the
corresponding devicetree overlay by adding something like this to
/boot/msdos/config.txt:

dtoverlay=rpi-poe-plus
dtparam=poe_fan_temp0=45000,dtparam=poe_fan_temp0_hyst=3000
dtparam=poe_fan_temp1=50000,dtparam=poe_fan_temp1_hyst=3000
dtparam=poe_fan_temp2=62000,dtparam=poe_fan_temp2_hyst=4000
dtparam=poe_fan_temp3=70000,dtparam=poe_fan_temp3_hyst=5000

Unfortunately this is where I'm stuck - the overlay is not present in the
final DTB given to the kernel so there isn't anything to hook a driver to.
Are dtoverlays supposed to work on FreeBSD?