I am restricting home directories for webhosting guest. It's quiet long since I have set it up so it's possible that following is not complete but try it and post your results...
ALL project quota stuff is managed from VE0, webhosting guest's private and root dir is on /data/v2/
disable vz quota in /etc/vz/vz.conf:
mount xfs with pquota:
/dev/yourdevice on /data/v2 type xfs (rw,noatime,nodiratime,logbufs=8,pquota)
/etc/projects:
2000:/data/v2/private/2002//home/www.server1.com
2001:/data/v2/private/2002//home/www.server2.com
2005:/data/v2/private/2002//home/www.server3.com
2006:/data/v2/private/2002//home/www.server4.com
2010:/data/v2/private/2002//home/www.server5.com
/etc/projid:
www.server1.com:2000
www.server2.com:2001
www.server3.com:2005
www.server4.com:2006
www.server5.com:2010
enabling quota for specific directory:
/usr/sbin/xfs_quota -x -c 'project -s 2010'
/usr/sbin/xfs_quota -x -c 'limit -p bsoft=500M bhard=520M 2010'