IR Flash test
OK, I’m shooting with an IR modified Canon 20D (830nm filter over the sensor) and a 50MM F1.4 with a 550EX with a Hoya IR (R72) filter taped over the head. F8 1/250S ISO 400 direct flash forced to 1/2 power at about 10 feet. It was over-exposed, but adequate for proof of concept, and if you start shooting in IR you will find that your meter reading is close to meaningless until after the shot. :-)
So, yes it works, but the filter still glows an ominous red when the flash is fired. I.E. they are going to notice when you flash.