My first experiment with discarding the barrier piece has worked out fine! A small hole in the fill port for the syringe needle does not seem to leak back into the mouthpiece at all! I am able to insert the needle through the mouthpiece hole and then into the fill port without removing the mouthpiece at all. In fact, it helps guide the needle into the fill port. It does have the effect of increasing air with each drag, but my wife and I actually like it better that way. It draws very easily, and if the air to vapor ratio is too high, I can just cover up a couple of the air holes at the seam where carto and battery meet.
The next step, using an air pump needle, or football needle as I call it, required larger holes in both the mouthpiece and the fill port. The fill port was no big deal to accomplish, and again I experienced no leakage back into the mouthpiece. In order to use the football needle without removing the mouthpiece, however, required using a 5/16" drill bit to enlarge the mouthpiece hole. This was no big deal to do, but it adds another task to the modification of each cartomizer. It also added more air to the mix, although not drastically so. I think it's a worthwhile mod for those who don't want to use a syringe and also don't want to remove the mouthpiece each time they refill using the "bottle mod" method. The biggest drawback to that modification is that the football needle doesn't fit onto all juice bottles. The ones I got from DIY were too big for it. It might be worthwhile to transfer the contents of those bottles to ones that have the smaller mouth, but I don't need that hassle.
For me, I think it stops at eliminating the barrier piece and shooting the syringe right through the mouthpiece. That eliminates the need to remove pieces each time you need a refill. That was my primary beef with using a syringe to refill them instead of the condom method or just dripping juice into the battery hole, both of which will work if you don't have tools at hand.