I work with several software packages (AutoCAD/Rhino/Sketchup/Blender) that all have "Mirror along line" capabilities or use plugins that do. I live by it.
Though I haven't been using Corel as much as I used to, I'm currently back using it. (2018) I seem to remember at some point I had a script or plugin that gave me mirror on line capabilities. Perhaps CADTools 5? Or maybe it was a how to video. Any suggestions?
You can do it with the Free Transform tool (hidden in the Pick tool flyout), but there seems to be a small angle deviation when mirroring.Make sure Snap to Objects is enabled (and Show Snap Location Marks).Select your object.Activate the Free Transform tool and select "Free angle reflection" in Property Bar.Then simply click and drag along the reflection line. Snapping will help you getting it right.I think this should do what you where asking for but like I said, the angle precision seems to be a bit so and so, at least in my small tests.
I think I have found a solution to the "drifting angle" issue that i mentioned, but it adds an extra step.Create an extra two point line along the reflection line and select it together with the object you want to mirror, and then mirror them both along the reflection line.This seems to make the angle perfect.