@rochemike sorry for the late reply!
If we already have an ISO on an SR, do we even need to mount the ISO for a network build of Centos/RHEL? Isn't... shouldn't it be possible for the ISO to be mounted by the new build VM as source for creating the new VM, instead of using network build?
That is exactly what packer-plugin-xenserver#56 accomplishes.
The new VM mounts the guest tools to the DVD - which I don't even want to use as I prefer to install a package afterwards. Is it possible to mount the RHEL/Centos ISO instead?
I believe the ISO and guest tools are treated the same -- both VBDs are attached as cds. If you can identify what is different about them in the UI or from the xe cli that would help me understand what you trying to describe.
As for what is possible, if what you want to configure is accessible via the XCP-ng / Xenserver API the packer plugin could be enhanced to perform that action. Hope this helps explain more on the potential possibilities.