Thanks @nliviu I’ve managed to cobble together something based on your steps and I’ve now got the build working.
I had some screwed up permissions that I had to fix before I could get libftdi and libusb installed and linked, but this post helped me out.
For reference, looks like what you’re doing is installing homebrew as x86, which based on this
post tells me it can coexist with the normal brew installation, but just needs to be referenced at /usr/local/bin/brew and it installs the x86 version of mos at /Users/<username>/.mos/bin/mos