Maybe grabbing an object from 3d view and dropping on the input field could work too and without cluttering.
that might be hard with continuous drag enabled
How would it determine the intent? pretty confusing if you moved the object first in 3dview, then it jumped back as you leave the area… and SterlingRoth has a strong point with continous grab
Nice to be of help 
This is a little different but could be nice if there was a key to activate “object drag/drop” function.
As an example: Everytime you hold CTRL or ALT and “drag” something you do not move the object. Instead you grab the “object entity” and can drop it into a field.
Like you can dragndrop textures, colors etc already. It would just be the same shortcut key to make it more consistent and it could work on many more things in the UI.
Way harder to implement though.
Is it in trunk @ideasman42 ? was it hotkeyed to E_KEY or? gonna check out latest builds at home during weekend and see if it´s there. 
if so ! the 5eur are yours BF foundation.
+1 for driver variables (as with After Effects style pickwhip), really nice idea for animation variables.