What about if you will have credentials set in WFA for those machines in the first place? Is that possible?
Another option is to create a table in the playground database (A section usable for users where you can create tables
with information, like user/pass) and in the workflow create a command to pull the information from the table and use it?
There's a way to get the currently connected user name in a command.
Just look at "Send Email" command for reference. Not sure if that's something useful for you, but it can be a key to pull
the password from the playground table...
Does those options make any sense?