Yes, quite easily. gmirror can be used to create mirrors across any number of partitions, slices, or whole disks.
I ran my home server with 3x 200 GB harddrives, partitioned as:
s1 10 GB for /
s2 1 GB for swap
s3 rest for ZFS
s1 was gmirror'd across all three drives.
s3 was added to a raidz zpool
There are a handful of different how-tos available online for using gmirror for the / partition/slice.
This is the one I used on my home servers, and two systems at work.