sed append in preseed/late_command

[Jack Bates <q34yzh@nottheoilrig.com>]

Hi,

How can I use sed in preseed/late_command to insert a new line, two 
lines after the line which contains "<!-- Volume definitions -->", in 
pam_mount.conf.xml?

Normally I'd use something like the following, but I think the sed 
append command must be terminated with a newline?

> in-target sed -i '/<!-- Volume definitions -->/ {
>     n
>     a <volume fstype="cifs" mountpoint="~/earth" 
path="students/%(USER)" pgrp="users" server="earth.lan" />
>   }' etc/security/pam_mount.conf.xml

Is there any way to express this script ^ without newline characters? Is 
there any way to embed newline characters in preseed/late_command value?