In the merge tool settings there is a checkbox called "merge without voxelization" or something very similar to that (I installed the German version during the last beta update by accident so I'm not sure about the exact English wording), that should do the trick.
Can't you use the cube primitive to boolean a 45 degree surface out of your voxel shape? You can rotate snap the boolean primitives to 45 degrees when holding shift while rotating it.
It only snaps to 45 degree angles though as far as I know, so manually defining the rotation increments would be quite useful indeed.
I've discovered 3d coat just recently and only played around with the curves and organic shapes so far.
This is my first hard surface test in 3d coat, just freestyling some scifi pistol as I try to figure out the tools.
Mainly done using the hide tool and primitives for boolean operations.