In another burst of "this should have been obvious in hindsight" I have discovered that you can't assign a physical serial port on a VMWare ESXi server to a serial port in a virtual machine that it's hosting. Evidently this is to cut down on the hardware dependency that would restrict VMs from having full mobility from host to host under VMotion. How that differs from the CDROM drive, which can be physically assigned to a VM, is unclear, but I understand the rationale.
Where does that leave me? I have a dependency to deal with: Proficy Workflow's OPC client can only make local calls, meaning the OPC servers it talks to have to be on the same host. I think what I will have to do is switch the serial I/O to another box and use a web interface to retrieve the data into the OPC server. It's a hack but it's a lot cheaper than going for a serial-over-IP solution since I've already got a bunch of beater hardware I can use for that purpose.