I am working with a device that can be triggered by momentarily shorting a pin to GND (I don’t have access to the internals, but I imagine the pin is normally pulled high). I would like to be able to trigger this device wirelessly using something like the XBee. Can I do this with some sort of configuration settings on the IO pins of the XBee (switching between ignore and normally low, maybe)? Would it be better to use a relay to short the pin to GND instead?
Yes. The digital I/O part of this project will do what you need:
http://www.faludi.com/2006/12/03/xbee-d … -with-adc/
Whether or not you need a relay depends on the nature of the device.