Can multiple PWR-01 series units be controlled via an RS232C interface?

Yes, multi-channel control is achievable through the virtual multi channel bus (VMCB) function.
The master unit is connected to a PC, and the slave units are connected via a LAN interface using a hub. You can connect and control up to 31 units.
RS232C, USB and LAN are the standard communication interfaces between the master unit and a PC.
For the details, read the communication interface manual – chapter of Multichannel Setup.