OK, let's cut to the chase. You're looking for a secondary tie in while blocking down a spar. One, to have a second attachment point, ( Tie In Twice Stupid , or T.I.T.S., as we all learned from the TCC.)
Two, to have a bail out line, preferably a one handed path to the ground in case of injury or anything else sudden and unexpected. This set up should be easy to reposition for production reasons, and easy to use in case of an emergency.
For these reasons, forget Ddrt, and your zigzag. Simplest method is some sort of choked srt tie in. Cheapest method is a hard locked Munter or figure eight, but those are methods I would only recommend to someone who has been using them for quite some time in various scenarios. Both are dynamic two handed methods that require a lot of practice to use smoothly and safely in any sort of high stress situation, which is the only time you will actually use your back up.
Which brings us to the best solution. Choked lifeline with a handled decender of some sort. My personal favorite is the Petzl RIG, due to the locked work position, and smooth cam. I'D is nice too. Grigri is ok, but tougher to use in a stressful situation.
Ddrt is still great for work positioning, but in this particular situation, more trouble than it's worth.
