Installed xenerver-core on top of vanilla CentOS 6.4 and I can verify that specifying alternative xenguest and qemu-dm values in the platform field of a VM is moot.
This functionality was introduced in djs55@63d41a8
I believe the problem might be that xenopsd-xenlight just doesn't care about xenguest (I tried a suspend).
Also, I don't think /opt/xensource remains the right place to install alternatives. Any docs about that?