Duck found this, but I haven't tried it yet:

"To enable SSH on an AirPort base station you just need to set the dbug property to 0x3000, like this:

python -m acp -t {ipv4-address} -p {password} --setprop dbug 0x3000

python -m acp -t {ipv4-address} -p {password} --reboot

Then connect to the device with the username root and whatever password you use to manage the device."

https://github.com/x56/airpyrt-tools/issues/1