Okay, been doing some work on this and started on both the editing UI and the backend. Guides are stored in a folder with a GUID for a name. The latest version is a Json file called latest.json, and previous versions are stored by unix time stamp in a sub folder called old, and indexed in versions.json. The php takes a GUID and loads the required data.
Current limitations:
May not work on Internet explorer! Haven't tested it myself yet but it uses html5 properties.
No editing of images.
No addition or removal of steps.
No saving.