If I want to make use of empty and otherwise wasted space on a standard 160x100 mm PCB, is there a way to
a) copy/paste a circuit layout and place the copy (copies) on the same board (have multiple identical layouts on one board)?
b) add one or more small layouts from other projects in the empty PCB space?
Is this possible with multi-layer layouts?
Maybe this link helps ?
As per the link, if you’re next step is gerbers and then fab, go for it.
I did it here for example with that small LDO test board (one is a copy rotated 180deg):
Also, as per link, possible, but you will loose the link to the schematics that way. No way around that in KiCAD for the time being. So do only once you’re ‘finished’ and next step are gerbers/fab (unless you’re OK with doing those steps as often as necessary).
PS: if I do panels I have all the schematics in that one project, but I didn’t do more than 2 layer boards so far and if needed could recreate the layout pretty fast… YMMV.
PPS: make sure the 100x160 really is the panel size available… above example was made while Elecrow still had that size for selection for an economical size. Once I was ready and got to order they changed the website and 100x160 is no more. I had to take 100x200 for my run 
Even if these mini-boards are in the same schematic, you need to join at least one net across to avoid them being seen as separate boards for electrical testing and charged as extra.
You can (crudely) create a panel from several pcb designs by starting PCBnew as a standalone application
(Windows, C:\Program Files\KiCad\bin\pcbnew.exe) and then use the menu File>Append Board,
but this will not keep links to any schematics either…
I’m not sure KiCad will ever be able to link easily multiple SCHs to a panel-merged / File.Append plot file.
Because you can have multiple C1’s R1’s and net names, how to correlate that ?
Best to manage as you describe, as a PLOT stage action, and saved to a TMP Board file, outside of any Design & ECO loops.
The merge steps could be sped with a master panel, that has courtyards ready to accept the append panels.
Just trying that, it seems the initial Block Append Cursor location has no initial user-control ?
(I tried Grid Origin and Drill & Place Offset, for no-change)
Looks like it uses the entity-centroid, which means texts etc can shift this about ?
However, once released, a box-select seems to use the bottom right select corner as the new mouse-origin.
- so I think a two-step can ‘append with control’ ?