Convert reed switch disconnection or SPST off to single "LOW" pulse
Sounds like capacitively coupling it might work well.  Reed switch goes from power a pull-down resistor and one side of the cap.  The other side of the cap goes to a pull-up resistor and the GPIO pin.  When the reed switch is closed, both sides of the cap will sit on the power rail.  When you open the switch, the pull down resistor will pull that side of the cap down.  If the cap is large enough and the resistors are properly sized, the GPIO pin will be pulled down as well until the cap can charge up.  When the switch is closed, the capacitor will discharge.  It might be a good idea to add a clamp diode to power on the GPIO pin side of the cap so the cap can discharge through the diode instead of the chip's ESD protection diodes.  You'll need to play around with the values of the resistors and capacitor to get a pulse of the right length, though the pull-down will have to be significantly smaller than the pull-up - I would suggest trying a 10k pull-down with a 100k pull-up.  To figure out about how long the pulse will be, calculate the time constant, tau = RC.  This will be approximately how long the pulse will be.  A 10k and 100k resistor with a 0.1 uF capacitor will give you a tau of about 110e3 * 0.1e-6 = 0.011 seconds.