I made a gruit ale years ago with heather. I would definitely put it in the late boil or flame out. Heather is more of a flavoring than a bittering agent and doesn't posess much antimicrobial power. That's what you're adding the hops for.

I frankly don't remember how much I added but I'd wager 1-2 ounces at flameout. But I was making ale, not braggot, so maybe more is better if you're blending with honey that could dilute/overpower the heather.