Packer and compressed ISO images
I tried the dumbest approach, i.e. just to plug the iso.xz file into iso_url and see what happens. Of course, this failed (even after manipulating the iso extension key). 

What I ended up doing --- knowing a lot about Bash scripting, and not much about Packer, Go and the like --- is to wrap packer calls in a script that downloads and decompresses the image, then calls Packer on the decompressed image.

I would be curious to know if there are more elegant approaches.