Really getting into “hack” territory here, unfortunately… You need more than one Momentary Button in order to do a PIN … I wrote a SmartApp for the Aeon Minimote (4 Buttons), so that’s worth considering ( Use Buttons As PIN Input ("Security Keypad") e.g., Aeon Aeotec Minimote, ZWN-SC7 Enerwave 7 Button; trigger to Routines, lock/unlock, arm/disarm, mode, lights ); but there is no SmartTiles Tile type that matches a 4-Button Minimote at this time.
Rest assured that the PIN per Tile or Sub-Dashboard is a common feature request and thus high on our list for consideration of inclusion in V6 or as added feature post V6 release. We’ve even enhanced the feature request to consider that perhaps the PIN is only required if the SHM is “Armed”.
You could certainly experiment with opening an interim web page (via a Link URL Tile) that requests a PIN before passing the call onto Smart Alarm. SmartTiles would probably do a pop-up PIN pad instead.
In the meantime, well, … please password, PIN, pattern, or fingerprint protect the tablet / phone, I guess. NB: The SmartThings native mobile App does not logout automatically and does not even logout if you change your password from another device!