You can angle them but they still have to have four sides.
The trick is to get the brushes as close to the original shape as you can before you make them displacements.
Here I've made a sort of arch. The displacements still sew and are four-sided, I've just moved those edges up where the tunnel needs to go. Then I very carefully shape the displacements just so they touch the tunnel wall.
You can also sew if the two displacements share a half-edge, as shown here.
EDIT: You can also just go the cheesy way--raise all the displacement vertices up and cram them all in the tunnel wall.