I'm having a similar issue.
What I have done, at the moment, is to draw the outlines on the Fab layer. This is very time consuming. However, this only needs to be once, and there are some tricks to make the effort go faster.
When I plot, I plot to PDF. However, in doing this, only one layer shows up on each individual Gerber. I thought about using GerbV to view the layers... Wait, let me do that!
Here is KiCad view of a section of Pcb showing Silkscreen layer:
Here is KiCad view of a section of Pcb showing Fab layer:
In GerbV the two layers at the same time:
Obviously there are some collisions between the layers as I did not have the method sorted out yet.
Like I said, it seems a lot of work to draw the physical parts, but I knew I wanted to create an assembly drawing with good details. I just had not worked through the entire process.
Using GerbV to Export both layers to a single PDF:
Okay, I have some idea how I'm going to solve this same issue for myself.