Yeah dropping the seed straight down is the best way I guess , even the wagtail relies on not much windy etc, but it doesn't have the drop point issues etc that a disc spinner does.I've often drilled grass seed like that with our old Accord DA 6M with the coulter springs disconnected. On a 90hp tractor on duals the ground pressures are low, the work rate is high and the seed spreads well. The metering is much more precise than a disc spreader as well.
.....I bet Mr vicon must've had a real as to how he could possibly build something that would broadcast in the same way as a mans arm would..