Support for drag and drop just made it into the RAP source code repository. You can use the same API as known from SWT and thus re-use even more code when single-sourcing RCP applications.
It will be available in the M4 build. In the meanwhile you can check out the sources from CVS and explore the new functionality online (go to the List page).
There are still some things left to do, for more details see this bug. We would be very happy to gain feedback about how your existing drag and drop code works on RAP. In case something doesn’t work as expected, please drop us a line or open a bug.