My experience is this:
Several years ago I had a blow off valve. It had an adjustable spring on it but it always was closed so tightly that letting off light throttle would surge a bit; or it was so loose that it was almost always on the verge of partially open, even at idle. Could never get it exactly right, but this is partly how I got some flames.
It was a GFB Stealth FX that you could vary between full VTA and full recirc, and the spring pressure was also continuously adjustable, as I mentioned.
I also was running an AccessPort with a Stage 2 off-the-shelf tune.
I used to get some flames occasionally.
If your BOV spring is setup on the loose side, you might be able to get it to happen.
For me it would pop a little if I would hold the throttle at a continuous point for a few seconds, maybe around 4k or 5k, and then let off completely. I know that it would happen on the street when under load, can't remember if it would when I was in neutral.
After revving like this a few times consecutively, I could almost predictably produce some flames out of the exhaust.
This was also running a 3" type loud-can straight-through turboback with no cats.
It was enough to be visible during daytime, I noticed this one day when I let someone else autocross my car.
Not like jet afterburners (not several feet or a prolonged flame), but enough to scare someone if they weren't expecting it.
As always, be careful, use utmost discretion, be considerate of pedestrians, other cars and neighbors, and I'm sure it's completely 105% illegal.
This is just for the sake of argument and how it could work.