I agree that it would be handy to have a drag-n-drop support for HDF5. The problem is that h5 files can store an arbitrary number of datasets that contain really any data from a simple table to n-dimensional image data, organized in a tree structure much like a file system tree. You'd have to make an automatic decision whether to load only the first or only the largest dataset, or all of them, and how many levels deep you want to go into the dataset structure where e.g. channels and time frames can be saved in different leaves.
On the last ImageJ/KNIME hackathon, I did some preliminary investigations how to add hdf5 read/write support to SCIFIO by modifying scifio-hdf5 (which currently only support writing files using the hdf5 flavor of Imaris), but I unfortunately didn't have time yet to continue this work.
If you have an h5 file type with recurring standard layout, e.g. from ilastik or from SVI Huygens, that you'd like to open by drag and drop, I suggest you create a small macro using Plugins > HDF5 > Scriptable load HDF5... and put the macro code into an ActionBar as a
<DnDAction> as a handy workaround.