Nope. First, the terrorists would have to have the equipment. Most of the guys catching and beheading folks are barely literate (Iraq continues to have a low literacy rate). Also, the transponder could operate on a classified frequency and use an encrypted transmission. The beheaders are barely literate thugs who are equipped with AK-47s and RPGs, not with elint equipment. These guys are different from the planners and implementors of the 9/11/2001 assault.
Besides, they could do it like LowJack, only send out a signal to turn the device on when said contract employee has been reported missing.