That's why I created [zeroxpos~] - so that it gives you the position of the 1st, 2nd, 3rd etc zero crossing within the block. Then you can use a [bang~] connected to a counter like [cup] within the block object to calculate the time from when the counter was last initialized, and work out when the zero-crossing happened.

It only works when audio is buffered into a table though - it's not true real-time but one block away.