After a lot of time I have finally succeeded in importing a custom texture to Alien Isolation. Just a simple replacement of a poster for now but I plan on taking it further and really pushing the limits to discover how far this can go.
Massive shout-out to Cra0kalo, without him this would not have been possible - he figured out the format of the textures a long time ago and if he had stuck with modding the game he could have achieved this long before I did!
In the coming days/weeks I hope to test the limitations of this custom texture method and eventually hope to develop a tool which will make the process a lot easier.