If you could dig up a FDT/dts file needed with the needed device tree entries then I would say maybe possible.

You would also need kernel support for the boards Pi 'hat' chip per say.
Like on an A/D electronics board I am working on, you need to add in support for the AD chip (if not supported) Like MPC3208 per my example.
--In addition to the FDT entries needed. I will note that Linux dts are similar if not the same on FreeBSD RPi.
So maybe you should ask the boards maker. They might have something from Linux work. They have a community page so ask there as well.
It will be tough to tackle. I would stick with what's supported on this.
Looks like they have the dts stuff here:

So then it comes down to this:
HifiBerry DAC HiFi pcm5102a-hifi
I assume its a PCM5102. Does FreeBSD support it? I don't see it so far.

The device tree overlays they use are going to need RPi3 for the DTO support on FreeBSD.
Here are instructions for the dts style setups like RPi 1 & 2.
Thanks for your insights Phishfry. I don't need a hifiberry specifically and I would just as soon consider another board (or ARM device) if I knew it was even unofficially supported by FreeBSD. The hifiberry just seemed like one of the cheapest options for an S/PDIF output from a raspberry pi. ALSA doesn't really cut it for me, so I may just patiently wait for some more options to open up.