Perhaps allow passwords...
Create a textbox for username and passwords and set the conditional formatting to
Username not equal to.... and list a load of usernames and then
Password not equal to.... and the passwords.
Then hide this control.
Or, if you prefer, do username is equal to..... hide this control. I do this for some of my infopath forms as I work for my local council so only certain users must access certain parts of the form.
This might work, it depends on what version of InfoPath you are using... Not sure if this answers your question or not but hope it helps. :p