So I managed to crack this one after searching through the sequoia-ptpy library code on Github. It appears that you can actually enable security on the Sequoia by using the PTP protocol and accessing an undocumented API call.
from ptpy import PTPy
camera = PTPy()
camera.set_device_prop_value("WifiPassphrase", "YourPassphraseHere") # Requires indent not shown due to forum formatting.
After restarting the Sequoia the WiFi will now start up automatically secured with WPA2-PSK using the passphrase you configured above.
Also I did an experiment and found that there is no restriction on maximum connected clients as mentioned in a previous reply above. I had both my PC and phone connected up to the Sequoia WiFi via the HTTP interface and I was able to configure and command the Sequoia from both devices simultaneously.