This post is just to document the location of a hotfix file that Microsoft Support fails to link to. They try to provide access from here: http://support.microsoft.com/kb/2842230. But the 'Hotfix Available' link doesn't provide access to the 64-bit version needed for Windows Server 2012.
I found the link eventually in a GitHub Repository: Packer-Windows. So thanks for that! The hotfix now allows WinRM to respect the configuration value for "MaxMemoryPerShellMB" instead of always allowing using the default value of 150Mb.
This means I can get on with provisioning MVC4 and IIS etc with Chef. Hopefully more on that to come.