This is a python API limit. To add it we must ask a new feature in Blender popup buttons. However consider that:
- Morph for each mouse movement is very CPU expensive
- You can modify quickly the last targets used by “applied targets” menu.
Of course, when possible, we add the option to have istant changes.
About the integration in Blender…Emk is right: MH is large download, and it interest only a part of Blender users. In future a lot of additional packages will be added, as eyes, hairs, expressions, facs, textures, bvh, pose, and, of course, targets. So the best is MH as separate plugin…