Yeah this can be done quite easily using an xml file as the source and using the custom property plug-in functionality. There is an example of a dropdown custom property in the SDK I think. The main issue I have is that I haven't found a way to make a dropdown list that is generic and reusable across sites as you can't define properties for a property. E.g. be able to define the xml file to load in admin mode, rather than coding it to load a specific xml file. Usually I just end up copying the property for each dropdown list I need in the site.