Sneeky way to get an OTD, release the parkbrake for a couple of seconds whilst holding the a/c with the footbrake. After a few seconds re-apply the parkbrake and it usually records that as the off blocks time, and in our case automatically sends it via ACARS.
Used to be like that, but we get an ACARS brakes release 10:00 (park brake off) Off blocks 10:02 (once you actually start moving). So your off blox time actually gets generated once the IRS senses movement.