I confirm that we don't offer this feature.
You might try to enforce secrecy by acting at a system level, in such a way that the password is in plain text but users other than the one running Lightstreamer Server cannot see it.
If useful, consider that the content of any element in the configuration file can be deferred to an environment variable through a $xxx notation (see the opening comment in the configuration file).
However, note that the configuration settings may also be reported in the Server log.