It would be great if the trim command could trim a hatch and curve at the same time. In Rhino 7 WIP 7.0.20091.16036 I have to run the trim command twice. Once to trim the curve and a second time to trim the hatch.
Try this technique instead.
Before you Hatch the curve, turn on History and then run Hatch.
The select the curve so it’s control points turn on, then move them where you want.
The History enabled Hatch will follow the changes you make in the boundary curve.