XDA forum member RustyGrom has already figured out how to install Windows 10 Technical Preview for phones onto non-approved Lumia devices. As always with Windows – it’s a simple registry switch.
From a high level this works by using FiddlerCore to intercept the traffic going to the Microsoft WPflights server that controls the Insider app and responds with our own custom data. The app accepts registry editing information from the web responses and acts upon that. This allows us to write stuff to select locations in the registry. In the case of the Windows 10 Preview, it appears to only look for your phone’s PhoneManufacturerModelName to decide if it should be offered previews. Windows Update also checks this value. Other devices like Samsungs or HTCs may need different settings.
The hack is still in its early stages, and really, don’t do this if you have no idea what you’re doing, but there are already reports of success.
XDA is a magical, magical place.