I would like to create a polyline in a plane that is not WCS. I am able to do so with the 3D polyline. My problem is that 3D polyline does not allow for a Bulge in the polyline.
Except if you're using the "_PLINE" command (with SendStringToExecute or SendCommand, ...) your program won't care about current UCS.
Use the Polyline Normal and Elevation properties or the TransformBy() method.