I can tell you that the boat I race on is not using metal shackles for the main halyard.

Further, we don't use covered lines either... we want to keep weight aloft to a minimum... so the halyards are bare core with a lump at the end that acts as a shackle. The cover is in place for about half the halyard length, then it is cut and sewn to the core at a one point. First time I saw it, I had doubts about how well it would work. But it has lasted two seasons of racing that also included a TransPac. BTW at the "end of the day," we "sky" the halyards with some cheap line; this keeps the core from being abused by UV.

See this for some other "racer tricks."

http://www.sailingworld.com/how-to/tech-review-10-cool-rigger-tricks

As far as knots... there is a halyard hitch that could also be used to secure a shackle.