The CLR's DEVPATH feature has always been broken and its brokeness varied over the years, but for my purposes it was a very useful feature. However with .NET .4.5 Microsoft decided to up the ante. If you have configured your .NET 4.5 runtime as a "developer installation", ngen will fail to generate native images for framework assemblies. This means that after Windows Update services mscorlib.dll, no native images will be used anymore.
Obviously you can work around this by editing the machine.config and re-running ngen, but that is a bit of a pain so decided to hack together a workaround.
The basic idea is to have a DLL that gets injected into (nearly) every process and patches
mscoree.dll to make CreateConfigStream read developer-machine.config instead of machine.config if the
DEVPATH environment variable is set.
The source and binaries are available here. Like the Microsoft DEVPATH code, this is untested so there is no warranty and use at your own risk. Bug reports are welcome of course.
There is no installer and there are no installation instructions, because frankly, if you don't know how to install it you probably shouldn't be doing so anyway..
The overhead is fairly low. If you don't have DEVPATH set the DLL will immediately unload again. The file size is only 3KB and in memory it will only use a single 4KB page.