Will Professional Services be able to install a custom OS on my server for me? (XenServer 5.0 - properly licensed).

Of course, knowing that there is no Operating System/software support from The Planet what-so-ever.

With the disclaimer you included here at the end (that we won't support the software), it shouldn't be a problem. Just create a quick support ticket requesting the service on the hardware you'd like to use and ask for a quote for the advanced services/professional services install.

Also, if you have to provide the media, send in a copy and not the original as we'll need to destroy it to comply with our various data handling policies.

Would there be any additional charges for this service? I'm looking to do the same.

As from what I believe, you just mail them the disc to install it, and order a KVM Rental so you can install it yourself.