Is there a way I can set all 14 cameras in my multicam setup to download their files to one single central folder?
Yes, but you need to ensure they end up with unique names. Two cameras could easily have files named IMG_0001.JPG. If you take many images between downloads, this could also happen on a single camera.
The download_images function accepts "substitution patterns" similar to the ones used by the imdl command in chdkptp. Use help imdl for information on that.
Download images adds a substition ${id} which is the number of the the camera.
The default download path (dst option to download images, see multicam.lua are around function mc:download_images) is
${id}/${subdir}/${name}
Which means a directory for the camera ID (e.g. 01,02 etc) the DCIM subdirectory from the camera (like 100CANON or 100___01 etc) followed by the image name (like IMG_0001.JPG)
If you just wanted to turn these into filenames without sudirectories, you could just swap the / to something else, like:
mc:download_images({dst='${id}_${subdir}_${name}'})
Then your files would be named like
01_100_01_IMG0001.JPG
edit:
If you want to download to somewhere other than the chdkptp directory, just put that at the start of the dst string, like
mc:download_images({dst='c:/pictures/${id}_${subdir}_${name}'})
You should use / instead of \ for directory names.