You can reslice the disk before creating the new boot environment and put the shareable file system on its own slice.  

For example, if the root (/) file system, /var, and /home are on the same slice, reconfigure the disk and put /home on its own slice. When you create any new boot environments, /home is shared with the new boot environment by default.

format(1M)

If you want to share a directory, the directory must be split off to its own slice. The directory is then a file system that can be shared with another boot environment. You can use the lucreate command with the -m option to create a new boot environment and split a directory off to its own slice. But, the new file system cannot yet be shared with the original boot environment. You need to run the lucreate command with the -m option again to create another boot environment. The two new boot environments can then share the directory.

For example, if you wanted to upgrade from the Solaris 9 release to the Solaris 10 8/07 release and share /home, you could run the lucreate command with the -m option. You could create a Solaris 9 release with /home as a separate file system on its own slice. Then run the lucreate command with the -m option again to duplicate that boot environment. This third boot environment can then be upgraded to the Solaris 10 8/07 release. /home is shared between the Solaris 9 and Solaris 10 8/07 releases.

For a description of shareable and critical file systems, see File System Types.