ReactOS MUST work on non-ACPI systems. On ACPI systems, ACPI will enumerate all your serial ports, even non-standard ones. On all systems (ACPI and non-ACPI), standard serial ports will be probed to test their existence.
I think, to correctly enumerate com ports in this situation, you need to remove enumeration code from pnp routines. What do you think about creating something like SerialEnumerateComPorts() routine and call it from DriverEntry? If you are not working on this now, I can try to implement it. The enumeration process is described in "Enumerating Legacy COM Ports" article from DDK.