How can I create a theatrical spark ejector?
Having been involved for longer than I care to remember in amateur productions building props, the one thing that strikes fear into a stage manager's heart is the mention of any form of naked flame on stage. 

I also go with the principle that you keep props simple - if anything can go wrong it will. So here is my suggestion for your "shorted switch".



A few notes: The 9V (PP3 type) battery is left in circuit. Notice there is no on/off switch. That's because "someone" will forget to switch it on one night. The 47k resistor (not a critical value by the way) limits how fast the large capacitor will charge, but once charged the circuit will only draw a very small 'leakage' current so a fresh battery will last for several weeks. The SWITCH is the actual switch operated by the actor. This will discharge the capacitor through a small speaker (giving a click sound) and overdrive the LED giving a bright flash (reduces the lifetime of the LED but not important for this application). Having discharged the series resistor will limit any further current and the LED will only have a very small glimmer.

Alternative solution. Hack a small flash gun from a non-reusable camera and wire the SWITCH to the trigger circuit.