Not with the current version of sysinstall (FreeBSD 6.x, 7.x), no. At least, not directly, using the sysinstall interface. You could probably script something up, though.
The
PC-BSD installer allows you to use gmirror (and I believe even ZFS, for non-/ partitions).
The
finstall project appears to enable gmirror (and ZFS??) during the install process.