Wow, well, what you want is called Emergency Management Services (EMS) redirection.
http://msdn.microsoft.com/en-us/library/ff542282(v=vs.85).aspx
And it works on modern versions of Windows.
In XP/2003, enabling it would look like
bootcfg /ems on /port com1 /baud 9600 /id 1
On Windows Vista/2008+ it is enabled by typing the following command where {default} is the OS entry in BCDedit:
BCDedit /bootems {default} ON
The following commands set the global EMS redirection settings to use COM2 and a baud rate of 115200, and enable EMS for the specified boot entry.
bcdedit /emssettings EMSPORT:2 EMSBAUDRATE:115200
Something that not many people know is that you can still connect to a Windows computer using EMS and perform a few administrative functions even after the computer has experienced a STOP/bugcheck/BSoD!
Powershell? I don’t know about that… I kinda’ doubt it, but who knows. The envisioned usage for EMS is after everything else has failed, but you can get a cmd.exe shell with EMS… and you can type powershell.exe from a Cmd.exe shell… so maybe.
That’s the best Microsoft-native thing you’re gonna get. Otherwise you are looking at third-party software for running a server over a COM or USB port.
The book Windows Server 2008 R2 Secrets By Orin Thomas has a decent section on EMS, which is actually freely Googlable.
Edit: Verified, Powershell is possible through EMS!
PS: You’re nuts!