what happens if I append to live (i.e. mounted) sqsh filesystems? Is this supported?

I've read some of the source code. And I've tried appending to mounted filesystems. But all I've figured out is: on the small sqsh files I've tried, the kernel didn't show any differences to before an append operation. (While I've expected the kernel (module) to crash or mksquashfs to err out).

What does the mksquashfs append operation do? Does it only *append* to the end of the file? In that case, can I somehow force the kernel to reload the file, so newly appended directory trees are visible? Or is this not possible at all, due to mkquashfs append having to rewrite global file headers?