This is really two problem reports in one, but both for chrooted behavior.
If running ISC dhcpd in a chroot, you cannot use hostnames in allocations.
ISC dhcpd running in a chroot needs access to a few files from /etc in order to
resolve host names from DNS.  Neither the port nor the rc script makes those
files available.  You also cannot use a configuration which is separated into
different files.

Fix: Copy those files from /etc to chroot /etc.
I believe the files needed are as follows:  

/etc/
	host.conf														
	hosts
	localtime
	resolv.conf

Here's two patches to the isc-dhcpd.sh rc script.  The first copies the above
files to the chroot at startup.  An rc variable could control this behavior.

The second allows definition of a directory to copy to the chroot at startup, 
generally for purposes of included configuration files.  This adds a new 
rc.conf variable, (I named it 'dhcpd_include_dir'), used like so:

dhcpd_include_dir="/usr/local/etc/dhcp/includes"  # dhcpd.conf includes
How-To-Repeat: Install port, insert configuration for a host like so:

host foo {
	hardware ethernet FIXME;
	fixed-address host.domain;
}

Insert line to include file in dhcpd.conf, attempt to run dhcpd in chroot and
note that file is not present in the chroot to be included.