As you've said the pictish ale recipe is long gone, we do a red that utilizes 1oz of heather tips at 10 minutes, it's quite different. I would say try that with your favorite ale and adjust from there, I wouldn't boil for longer than that as they are quite strong. Also most "herbs" create a harsh bitter flavor when boiled for long periods.
Edit: We've recently made a braggot with heather that is probably closer to the pictish ale than the red, it all depends on your tastes though.
Newer, better, more streamlined sig as per the forum police.