Trying to solve my other problem with removing the DVD drive, I upgraded to the latest version of the PSHyperV libraries. Since then, creating a drive with a differencing disk doesn't work anymore.
$vm = Get-VM -vm <vmname> -server <server>
Add-VMNewHardDisk –vm $vm -controllerID 0 -lun 0 -vhdpath <diff.vhd> -parent <parent.vhd>
result is a drive with <diff.vhd> as primary vhd, and no link to the <parent.vhd> file...