After posting images and video of Windows RT 8.1 running on Lumia 640 XL the dev behind this hack has now posted the installation tutorial to help others. The dev has earlier been able to enable touch and internet. In fact, Lumia 640 XL handles Windows RT 8.1 so well that it surprised everyone. Check the hands-on video by clicking here.
In his latest hack, the dev has even downloaded and installed Windows 10 apps on Luma 640 XL running Windows RT 8.1 from Windows Store. Check some of the images of Windows 10 apps running on the Lumia 640 XL.
Before trying to install Windows RT 8.1 on Lumia 640 XL you need to know what doesn’t work and that some steps are quite risky. So, do it at your own risk.
While this project is not totally finished (e.g. No Battery status/charging unknown, no cellular, no audio), I decided to post this thread as many people asked me about the tutorial. This is just a brief tutorial, you need to have plenty of time on this to make this happen. Be careful as some steps are very dangerous.