Unable to mount remote directory using SSHFS
Turns out it was just a case of RTFM. I needed to install the kernel extension too, like so:

sudo /bin/cp -RfX /usr/local/opt/osxfuse/Library/Filesystems/osxfusefs.fs /Library/Filesystems
sudo chmod +s /Library/Filesystems/osxfusefs.fs/Support/load_osxfusefs