I have done some research too in the last couple of days.
So I figured I'd share my findings and contribute a bit.
1. Apparently MSI once had a working prototype of a PCI-E to 2* MXM
We are trying to achieve the opposite and things have changed quite a bit over the last couple of years but it shows, that PCI-E and MXM share many things, MXM just has some additional features.
Back then it was MXM Rev. 2 I believe.
2. Some Chinese site is selling PCI-E to 1* MXM Adapter Cards.
This demonstrates again, that an adapter is possible. This card doesn't seem to utilize the full bandwidth tho.
3. Leap Computing Inc. requested a patent regarding a "Mxm graphics card adapter"
Initially I thought they made a MXM to PCI-E adapter but it seems like another PCI-E to n* MXM Adapter.
4. Some guy seems to have a kind of working adapter.
He has made a short YouTube video demonstrating it.
https://www.youtube.com/watch?v=0YzkpITCiUY
I made a Google Drive directory where I will save all of the stuff regarding this topic. Everybody is free to view / download the files.
https://drive.google.com/open?id=0B6KCAsL5ElLUYXFmdnFoZEozSkk
Now I hope to get hold of some PCB design files and let them be printed.
If this works then I'll try to add additional functionalities. I'd try to get a video signal back into the laptop, so it'd be possible to use the internal display again / too.