I have settled on the copying and pasting solution for now. What I need is for the screen to flash black and white every 500 ms, with a message for the participant to "click continue to end session" (I want them to think there's some kind of programming error). The screen will go black. Then, 30 seconds later, I want the cycle to start again. If they don't click continue, I want it to keep flashing for about 10 minutes. They'll be working on a paper & pencil task next to the computer.

I've got it working, but I'll have to copy and paste a lot of lines to make the flashing last 10 minutes. It'll work fine if that's what I have to do.