Answer by Jason 15

The only way I've found so far is to put your file(s) in the root of the Plugins directory and append the '.dll' extension to them, even if they're not dlls. For example, if you have text.xml then name it text.xml.dll. I had to load an XML file in an external .dll plugin (so I couldn't use the Unity Resources or TextAsset class). This is the solution I used and it works fine, if a little hack-y.