Thanks :-)
Have a look at src/app/image/image.model.js. (image = either an instance or a single frame from multiframe instance)
You can simply access dicom tags via "image.tags". Not sure it would work well with multiframe instance in this case though.
If you have a look at image-plugins/base-tools.class.js samples for instance, you can access the viewport and its current image via the getImage method (see viewport.model.js). You can then listen to image changes if you need to.
Have you a good understanding of the angularjs framework? This part of the code isn't massively documented and angularjs may be counterintuitive, especially concerning directives.