I looked into this a bit... The "encr" string only applies to Basic Authentication. I don't think there is anyway to use that for general string encryption.
I didn't find anything else to indicate this is natively supported. You would have to include a c++ library/code it yourself. But then all the code & data needed to decrypt would have to be in place on each QA player PC, making it all but useless. You could possibly precompile something to a DLL, but then I'm not sure how to get QA Load to work with that...
The best I've come up with that I use is reading the login data from a file and then I make sure to clear the datapool files from each player when I'm done. Sounds like that wouldn't work for you if you are sharing the scripts/data with others.
Of course your scenario wouldn't work then either because someone using your scripts/data files could set it up to use a proxy on a player PC and then capture the unecrypted PW as it is submitted by the player agent.
I think you are down to having to reset the PW after each test or something...
A problem is a difference between what is perceived and what is desired, that
we want to reduce (Dewey 1933)
To overcome this problem we have written two functions (using basic C++), one for encrption and another for decryption. We input the encrypted password in datapool file and use the decryption function in the script to decrypt the string and use it.