Your split mainsheet will exert sideways force to the extent that one of the split ends is unloaded, so the other end takes over. Unlike the case of my first rig, there's no longer a requirement that both sides pull equally.
The block would not have to be far off center to unload one of the ends completely.
However, when you look at it, the minute the boom is at center, the two split ends have the same length and start to both pull. So the true center is probably also not reached, but I be happy to believe that you can get a bit closer.
Phill's rig (running/fixed triangle + end-book tackle) also allows one side to take more pull than the other (whenever the free running triangle is turned into a fixed one by hooking the knot over the hook under the transom). The one difference compared to the split mainsheet is that the latter changes geometry in a different way as the end of the boom rises: you simply get a taller triangle, whereas with a fixed triangle you end up with the end-boom tackle adding some distance to leeward.
However, with Phill's rig, you could add additional hooks under your transom and pull the lower end of the tackle to windward, or use some other arrangement that lets you adjust that triangle.
The question is, can the DS be sailed effectively with the main sheeted that far in? If yes, then your search for a solution in that direction makes sense, if not, perhaps it's simple not an issue.