Thanks, that info got me going. Didn't realize it could be that easy. I created a .reg file and after some tweaking my built-in COM ports are working great. I'll share the content of my .reg file in case it may be helpful to anyone else coming across this thread. NOTE: It turns out that the UARTs on my board are clocked faster than the default assumed by serial.sys, so that is why I have to specify the ClockRate value. YMMV, and you can likely omit it.
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Serial\Parameters\Serial2]
"DosDevices"="COM3"
"Interrupt"=dword:00000006
"InterruptStatus"=dword:00000580
"MaskInverted"=dword:00000000
"PortIndex"=dword:00000001
"PortAddress"=dword:00000600
"ClockRate"=dword:E10000
"LegacyDiscovered"=dword:00000000
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Serial\Parameters\Serial3]
"DosDevices"="COM4"
"Interrupt"=dword:00000006
"InterruptStatus"=dword:00000580
"MaskInverted"=dword:00000000
"PortIndex"=dword:00000002
"PortAddress"=dword:00000608
"ClockRate"=dword:E10000
"LegacyDiscovered"=dword:00000000
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Serial\Parameters\Serial4]
"DosDevices"="COM5"
"Interrupt"=dword:00000006
"InterruptStatus"=dword:00000580
"MaskInverted"=dword:00000000
"PortIndex"=dword:00000005
"PortAddress"=dword:00000620
"ClockRate"=dword:E10000
"LegacyDiscovered"=dword:00000000
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Serial\Parameters\Serial5]
"DosDevices"="COM6"
"Interrupt"=dword:00000006
"InterruptStatus"=dword:00000580
"MaskInverted"=dword:00000000
"PortIndex"=dword:00000006
"PortAddress"=dword:00000628
"ClockRate"=dword:E10000
"LegacyDiscovered"=dword:00000000
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Serial\Parameters\Serial6]
"DosDevices"="COM7"
"Interrupt"=dword:00000006
"InterruptStatus"=dword:00000580
"MaskInverted"=dword:00000000
"PortIndex"=dword:00000007
"PortAddress"=dword:00000630
"ClockRate"=dword:E10000
"LegacyDiscovered"=dword:00000000