From Brooke Meyer review of the RF -603II (similar problem) -
"9. Stupidly put Yongnou 603 II C1 Canon Transceiver on K5IIs body backwards. Spring loaded pin drops into locking hole.
10. Slap forehead and destructively disassemble Yongnou 603 II C1 Canon Transceiver on K5IIs body."
If you can get a right angled screwdriver onto the base screws you may be able to separate base from the the body of the controller and possibly access the pin that has fallen into the locking hole used by the Pentax flash.
Sorry I can't help further....