Okay we actually came up with a good solution - we only want to use the desktop app inhouse - so we came up with a bit of a different solution
in the app config for the desktop app we have added the machine key, the membership provider, and the connection string then we add a reference to system.web then i can simply access the aspnetsecurity.memebership user class and all its methods
so i can call getpassword - it works for encrypted passwords but of course not for hashed ones
I can send someone the code if they are interested.