What, I'm not the first person to consider using string, to pull things together? Outrageous! And here I was already on my way to the patent office.
The washer idea seems a good one; they would tend to cant sideways and lock into place. I was so intent on keeping weight low as possible, I stitched loops. Loops just large enough to make it onto the ends of the poles, but not large enough to slip up onto the poles. Also, stitching the intersection of the X made the device easy to use; that stopped any potential "runaway" effect. If I were to do it again, I'd use different colors of twine for the legs of the X, so you could tell easily which line was which. If you don't lay it out as an X you make a sort of )( shape, with a lot of stress on the stitching at the intersection, so you definitely want the lines to cross. I resolved this by sharpie-ing one of the lines to distinguish it, but having two colors would be more professional.
I've long considered doing this, since I use a home-made polycryo groundsheet that has no grommets and thus is no help in capturing the pole ends. I like that you can get the poles bent to shape and hold them there, and then futz with the placement, before erecting the tent. I'm also more likely to pitch this as desert shade during lunch, now. It was too much work, previously.
About erecting the tent INSIDE the fly: Mixed results. I found attaching the base of the inner, and clipping the inner into place, straightforward. What was difficult for me was attaching the grommets at the top of the inner to the cross pole. That gives me issues under ideal conditions (it's so tight), and I gave up even trying, with the fly already up. So one side of the inner's grommet made it onto the top cross pole, and the other side got a Nite Ize Gear Tie holding it within an inch or two of where it should have been. Major cheat. Yeah, I agree, I won't be erecting the tent fly-first, unless necessary. It complicates things. BUT, I would definitely do it if I got caught in a downpour.
I'll also be packing my tent differently now, with the fly and the poles readily accessible. (Previously they were wrapped to the inside.)