Don’t think there’s a way to do it with “chain.32”. I found other references to “pxechain.com” from the linux utilities.
I added the following and it seemed to start booting but unfortunately didn’t boot WDS:

LABEL Windows Deployment Services
kernel pxechain.com
append IPADDRESSGOESHERE::boot\x64\pxeboot.n12
MENU LABEL Windows Deployment Services
TEXT HELP
Loads the Windows Deployment Services Imaging Environment.
ENDTEXT

So I’m going to give up on this one and do what everyone else seems to have done.
Which is to put the linux boot files onto the WDS location and modify from there instead.
Reference: [url]http://www.syslinux.org/wiki/index.php/WDSLINUX[/url]

Actually, someone found this as well (multi-PXE boot) which I might try out first:
[url]http://www.vcritical.com/2011/06/peaceful-coexistence-wds-and-linux-pxe-servers/[/url]