Hey guys,
I've created this patch which addresses some issues I had when trying to get my encrypted volumes to be opened with the --allow-discards option on boot.
My root partition is unencrypted, so I didn't need to do anything with an initrd; this patch works for volumes that are opened later in system startup.
The key insight I gained is that /etc/crypttab is read by /etc/rc.d/rc.S during boot, and so the file format is really just dependent on whatever that script is doing with it.
I'm just posting here in case my experience may be helpful to others!
https://gist.github.com/nathanbw/7279227