The perceus initramfs.img is a standard initrd. To modify an existing one use these steps:

1. Make a backup copy

[root@perceus tftp]# cp initramfs.img initramfs.img.backup

2. Create a directory to hold the working initramfs contents and unpack the initramfs into it

[root@perceus tftp]# mkdir myinitramfsdir
[root@perceus tftp]# cd myinitramfsdir/
[root@perceus myinitramfsdir]# zcat ../initramfs.img | cpio -idmv
dev
dev/pts
dev/pts/0
dev/pts/1

3. Modify whatever you wish to modify.

4. Repackage the directory into an initramfs image

[root@perceus myinitramfsdir]# find . | cpio -H newc -o > ../mynewinitramfs.img
28633 blocks
[root@perceus myinitramfsdir]# cd ..
[root@perceus tftp]# ls -al mynewinitramfs.img 
-rw-r--r-- 1 root root 14660096 Jul  8 13:50 mynewinitramfs.img

5. Point perceus toward your new initramfs image by modifying pxelinux.cfg/default, adding custom pxe configs for specific machines, or making initramfs.img a link pointing to your new initramfs image. Lot's of ways to skin the cat.