Force Paste is an Alfred 2 Workflow that can be used to "paste" text into stubborn password prompts by sending a keypress event for each character in the contents of the clipboard.
Double click the Force Paste.alfredworkflow file and follow the prompts in Alfred 2 to import the workflow.
The workflow is set to pass the contents of the clipboard to the AppleScript function.
on alfred_script(q)
delay 0.1
repeat with n in (characters of q)
tell application "System Events" to keystroke n
delay 0.1
end repeat
end alfred_script