I had a similar requirement for a virtual Cisco wireless controller. You can try the following:
1. Create a port group for accessing the serial console. You can choose a disconnected NIC too since it wont communicate ouside of the ESXi.
2. Add an NIC to the VM from this port group.
3. Add a serial port to the VM. Choose the 'Use network' option and then choose the 'VM listens for connection option'.
4. Make sure the 'VM serial port connected to vSPC' is checked in ESXi firewall.
Let me know how it goes.
Pavit