Are there examples in the Civil War were trenches were dug so carriages could be lowered to a steeper degree giving the projectile a higher angle of trajectory?
I do remember reading an account where a battery had dug holes under the trail of the guns to get a higher trajectory, but right now I can't recall where it was done or the source.
Shelby Foote mentions that at the Battle of McDowell, General Milroy dug a trench of sorts to dip his cannon into, so they could fire at a higher trajectory.
Not sure if this meets your definition of "trench," since it was a field engagement, and not "trench warfare."