While I don't like the very idea of "thin provisioning" too much, I'm sure a lot of people (including me) would love to see MD just pass along the discard requests, which to my knowledge it doesn't do, yet.
We run a number of filesystems on SSDs and would like to use MD RAID10 on top of them - which is currently not possible because we would lose the very import feature of being able to keep the SSDs performance by periodically wiping the unused sectors.
Passing along the discards should be much easier to implement than what you suggested, "DM" already does it. Please consider this, too :-)
