I've used Juniper a good bit with varying results. I have always been intrigued with the way things used to be done and the twist and turns that got process to where the are today. My understanding is Juniper was originally used in Scandinavian countries.
I've used them at all different times in the boil but have found the full length of the boil to be best, for me at least. There is good bittering potential but the latter additions don't seem to do much as for as aroma or taste. Seeping in the primary does good though. I guess the etoh leeches out the resin and gives that gin like taste & aroma but more piney and woody like.
I lined the mash tun with red cedar branches once because of an old recipe. It described how the mash vessel, a big wood barrel, would be lined with Juniper branches to create a filter bed, filled with water and malt grains, and then the brewers would start filling the vessel with red hot rocks until the steam was coming off the water steadily. After a period of time a plug would be pulled on the bottom to drain the wort into a boil vessel with the branches acting as a filter for the grains and then more hot rocks added to bring to a boil where spices and herbs would be added.
The time I tried it there was a very slight flavor but it did do a decent job of filtering. I did have some braid in there too
Wish I had a big ol' wood barrel.