That's because what the slicing tool does is load up an internalized test page, using the Ext JS framework, and looks at the controls in the framework that it knows need slicing, then generates images accordingly.
I don't believe it's designed to do slicing outside the framework, because it wouldn't have any idea which things you need sliced.
Yes, of course. This have to result into something like 'sencha slicer my-class-1 my-class2'
During my current explorations about sass/compass (and sencha sdk) I'd thought that could be a good starting point. However, I could not found any good starting point except the definition of the slicer command itself (the Module.js).
Another possible way could be a special sass plugin (in combination with phantomjs), but that is currently only a quick flash. I don't know whether this actually works.