I play in two regular tourneys that each use a slightly different rebuy technique. (they're both $10 buy-in)
One has a single $10 addon available to any player, in the first 4 levels, regardless of chip count.
The other has unlimited $5 rebuys thru the first 4 levels, plus 2 $5 add-ons at the end of the 4th level. You must be empty to do a rebuy, but everyone can take the add-on's regardless of chipcount.

Needless to say, the $5 unlimited rebuy is a very loose game and can be frustrating to the better players. (But the pots are pretty nice!)

Oh, and I have never played nor heard of a tourney that let you sit out hands and then buy back in. You have zero chips- you buy back in then and there or you're done. Skipping hands is not an option.