The ramdrive is defined as a physical drive. It makes no sense being unavailable.
Well I guess it's how it being defined and done so in such a manner that VMware will not work with it.
As a workaround, if I was in that situation, here is what I do... I'd create a virtual hard disk stored on the RAM Drive and then always start the VM from a Script that copy copies the virtual hard disk file from the hard drive to the RAM Drive and then starts the Virtual Machine. This way with the virtual hard disk being used for temporary files will be much faster then using a virtual disk on a Rotational HDD. The Script could be appropriately coded to ensure the .vmdk is present on the RAM Disk before starting the VM or it can be scripted to be placed there at login to the Host so if you want to start the VM from the VMware Player GUI, whatever... there's always more then one way! 