I managed to make the Panelizer work and created a prototype of a 2x2 macro PCB:
However, I see some strange corner-things (extra circles outside of the PCB) and apparently these are present also in the original Gerber design; this is what JLCPCB …
Update #1: the JLCPCB website offers this feature ("panel"), but it is pricey.
Update #2: there is a tool for this, http://blog.thisisnotrocketscience.nl/projects/pcb-panelizer/, but I was unable to load the MiniMO gerber files into it so far.