In your custom page, u can access it thru window.parent.top.location.hash.replace('#AssetEditorPlace:','');
ie. var uuid = window.parent.top.location.hash.replace('#AssetEditorPlace:','');
The above solution probably works, but it is dependent on Guvnor 5.x. If you switch to jBPM 6 I'm pretty sure you wont be able to carry it over. You could ask Designer itself what the UUID is that it has currently open which is compatible across versions (2.4 and the upcoming 6.0):
- locate the frame that Designer is opened in and ask for ORYX.UUID which contains that value, for example: window.frames.ORYX.UUID
Designer has a very good public JS api from which you can get a lot more than just the UUID (you can get the process json, svg, bpmn2, manipulate parts of the process canvas, etc etc).