Thanks for the reply, but I'm not trying to restore *to* a read-only file system, I'm trying to restore *from* a read-only file system.
/home/USERNAME is on a normal, writable file system.
/backup/.zfs/snapshot/2016-07-10/accounts/USERNAME is an incremental backup that is on a read-only file system (we implement daily incrementals using ZFS snapshots).
The restorepkg command isn't having trouble writing to /home/USERNAME, the problem is that during the process it creates temporary files in the same location that the backup is located at (/backup/.zfs/snapshot/2016-07-10/accounts/USERNAME in this example). It shouldn't need to do that, since it's copying data anyway, it should be possible to use the *destination* for the temporary files, or even a different location entirely.
I was just hoping that there was an undocumented command line switch, or maybe an environment variable that could be set to tell it to use a specific path for temp files while restoring.
In any case, it's a non-issue now. Using unionfs does the trick and now our clients have daily incremental backups that go back months without using huge amounts of disk space (thanks to ZFS)
