I modelled a new part for the slider block, where I could mount a gear train connecting the main driver to the spool holder. I did a quick test after printing the part and the corresponding gears. The part worked ok, but now I need to print more links so I can test with a longer chain. The extra bulk will probably require a longer part, and maybe an update to it's angle for at better alignment with the guides.
Next I need to model the gears needed to connect the other spool.