The Rx:plain/secure button (or menuitem) shows only
instantaneous status of last received message. No
information is kept in the scrolled area for any other
message nor in the log files if you record your messages.
Could it be possible to have an icon (like a
lock/key/whatever) that would be added directly to the
beginning of the message (i.e. part of the text)? It
could be using a string like (LOCK) en (CRYPT) that
would be replaced by relevant icon just in conversation
window, like all other emoticons and would look nicely
in log files ;-)
By the way, changing the icon of encryption-capable
buddies in the buddy list could be a nice addon, like
requested in an old message for AIM.
Hope you find this idea interesting.