Fill in system password dialogs from 1Password

system-pass-paster.applescript

-- Change E27 to a unique string that will find your system password in 1Password
do shell script "open x-onepassword-helper://search/E27"

-- Give 1Password a smidge of time to open
delay 0.5

-- Copy the password to the clipboard (1Password will auto clear it after 90 seconds)
tell application "System Events" to keystroke "c" using {shift down, command down}

-- Give 1Password a smidge of time to copy
delay 0.5

-- Focus the system password dialog
tell application "SecurityAgent" to activate

-- Open ForcePaste, an app that types out your clipboard using virtual keyboard events
-- Secure system dialogs don't allow you to paste into them normally
do shell script "open /Applications/Force\\ Paste.app"

-- Delay a smidge as Alfred loses focus when activating via a Workflow
delay 0.5

-- Uncomment this line to press enter after pasting.
-- I didn't do this in case I accidently activated this while in Messages, Slack, etc.
-- There's no safeguard to prevent it from sending your system password to whoever you're chatting with.
-- tell application "System Events" to keystroke return

You'll need to download Force Paste to your /Applications.

For convenience, I created an Alfred Workflow to launch this AppleScript via a global keyboard shortcut.

It didn't want to run from within Alfred so I saved this as an AppleScript Application (an option in Script Editor's save dialog) and gave it accessibility rights in Security.prefPane.