The rope going through the biner as a simple tending device works, but it's far from optimal. I tried it for the less bulkiness on the bridge. It was a pain because of the friction rope-biner and rope-prussik's eyes. If you tend the slack with a Pantin, you can go with it, just a stronger push. But when you have to tend the slack by hand, like for the return from a limb walk or changing the setting during the climb, you really feel like fighting it. A small pulley is far better.
With a shackle and its rough surface, it would be even worse.
Don't expect the liquid tape lasts long, between the friction from the rope and the high pressure point with the ring. And be prepared to see your ring's coating dammaged by the contact of the shackle, by the pin, but by the sides too.
With the shackle, be careful to not put it in a unscrewing potential situation.
To get a mounting shorter than with a biner, I used a screw link, the triangular version, as a central point on a previous saddle. To replace the biner on the HitchHiker, I took a bit bigger one. The surface is smooth, rope/web sling friendly, not much to catch by everything on the side (unlike the shackle), no risk to unscrew it by the rope's friction, rated WLL 2500lbs for a tad less than 1/2" (steel diameter), ...