I'm wondering how to get particles to inherit the velocity (at birth) of the source object? So it's as if the emitting geometry, were it to be spinning, would throw off the particle. They'd just drop if it wasn't spinning.

Thanks

If Initial velocity is set to Use inherited velocity under Attributes tab. Then source velocity will get picked.

Make sure you have v attribute on source geo. If not use trail sop to compute velocity.

Excellent, works perfectly