I thought I'd share in case anyone is unlucky enough to have this happen at some point. If you use credssp as authentication enum when connecting to a remote PowerShell session, your user token is marked as "impersonate", this means that you are part of the NETWORK USER group which means no visibility of WCF named pipes as the SID 5-1-5-2 (more commonly known as the Network SID) is denied access to named pipes. Here's an enlightening article on the subject: KennyW's blog.
The process I was remotely starting with my PowerShell commandlet started child processes that were communicating over named pipes :(
So in the end had to psexec (sysinternals) as system to get that process to execute locally within the remote session. Not elegant but it was all I had available.
Also thanks to Keith Hill for his help with the PowerShell troubleshooting!
My blog article detailing my findings - http://josephkirwin.com/2013/05/06/the-named-pipes-had-a-party-and-imposters-were-not-invited/