Well I don't know how exactly your system/your installation looks like but I guess it works like this:
You log in with your roundcube user. The roundcube application encrypts your given password and compares the given password with the one on the system. If the password is correct, the session gets started and the encrypted password is stored in your session and the value, you're trying to read out.
I guess the only possibility to have that what you want is to make two different password-vars after loggin in: one which gets encrypted to compare with the password in the system and one which saved the given password in cleartext.
Unfortunately that goes too deep into the roundcube core - at least for me.
If anyone has another idea or I'm mistaken, give it a shot ^^