Instead of the string you could also use an electrical connection to a sensor light through a fusible link.
They make links designed to melt at 165 degrees (I am sure other temps are available) to hold up fire curtains on computer server rooms.
If no fire the curtains restrict airflow to make the server cooling system work properly, but you want the curtains to drop at the first hint of a fire so the fire suppression system can flood the rooms unrestricted by the curtains.
http://www.jrclancy.com/firesafety-fusiblelinks.aspSee the bottom item Fusible Link 016-7519 It is composed of two metal tabs soldered together with a low melting point solder which comes unglued at 165 deg F.
Similar links are used in self closing fire doors, could be used to release a fuel line valve to automatically close fuel line off in case of fire etc.
http://www.globetechnologies.com/fire-doors-windows.phpThe B and K type links have temperature limits up near 350-450 degrees if you were worried about low temp false alarms due to heat soak.
Just connect a wire to both ends of the tab and when it gets hot the link will break opening the electrical connection.
As far as the view port is concerned I would go with tempered glass or wired glass (both are used in fire doors and similar high temp applications).
You can also partially shield the view glass by placing a piece of metal screen on the hot side of the glass to conduct heat away from the hot gasses before they get to the glass (often done in wood burning stoves).