This is not directly possible at the moment.
But our backup tool can execute hooks at different backup stages, so one way would be to make such a hook script and split it there "manually" and then upload the single files as usual.
Just as an idea.
A basic example hook script can be found at:
/usr/share/doc/pve-manager/examples/vzdump-hook-script.pl
You can configure to call this script globally in: /etc/vzdump.conf
Or per vzdump call with the '--script' switch.
man vzdump
gives a little more information about all this.