Well, you're already well outside the bounds of supported behavior, so what can it hurt...
You can use FieldUtils (as done here) to read the inner com.inductiveautomation.ignition.gateway.secrets.Plaintext field from a PyPlainText. The field name is "wrapped".
As in, something like this:
pySecret = system.security.readSecretValue("abc", "xyz")
javaSecret = FieldUtils.readField(pySecret, "wrapped", True)
self.instance.setProfilePassword(javaSecret) # new for 8.3