While trying to install FreeBSD via the bootonly image resulted in

mount failed with error 19

But using

• dhcp/tftp servers on Wheezy
• packages syslinux, syslinux-common and pxeboot from Jessie
• mfsBSD SE USB memstick Image

I was able to install FreeBSD via netboot with ZFS on /.

Copy syslinux files to the tftp root:

cp -r /usr/lib/syslinux/modules/bios /var/lib/tftpboot/syslinux

Download mfsBSD image, gzip and copy to /var/lib/tftpboot/syslinux/freebsd

Configure pxelinux:

label freebsd
menu label freebsd
kernel memdisk
initrd freebsd/mfsbsd-se-10.0-RELEASE-i386.img.gz

After booting, look for disk and install:

geom disk list

Geom name: ada0
Providers:
1. Name: ada0

zfsinstall -d /dev/ada0 -s 256M -u http://www1.de.freebsd.org/freebsd/releases/i386/10.3-RELEASE/