So, you wan't to have a block device in your LXC? Why?
In theory, this could work if you play around with LXC internals (nothing in PVE directly), in which you create a ZFS volume and determine the major and minor number of the device, then you add an LXC setting to allow access to that major/minor conbination and create the blockdevice yourself in your container. It could work, but involves a lot of manual intervention and it may not be reboot-safe.