If I'm not mistaken, the main concern about these tacks is that they have a tendency to crack, resulting in the possibility of the crack propagating into the base metal, especially in Cyclic stress conditions. I ran acroos this problem while working on cylindrical legs for Offshore Jack-Up Oil Rigs. We seal welded the backing straps as a solution.
As you are quoting from D1.5, I assume you are working on a bridge. If this weldment sees a lot of stress, I recommend that you seal weld as well, if possible. If it's not possible, can you re-design to allow accessibility? Or perhaps revise the WPS to eliminate the backing strap all together? If backing is a must, consider the stick on ceramic tape for backing. This will still require a WPS change, but it's an option.
There is an option. D1.5 sec 3.3.7.5 says the tacks should be placed within the weld joint so they will be remelted and become part of the final weld. 3.3.7.6 applies only if 3.3.7.5 is not done.
That may not be the easiest way to do it, but usually not too hard if you watch your joint prep.
CHGuilford