That combination might work well. Squeak will highlight projects you are about to drop into, but it doesn't always help very much because project windows are so small -- you often can't see the highlight! With translucency, you could see the highlight.
Alternatively, the highlights could be drawn in an overlay plane (just like the HandMorph - maybe even the Hand itself should draw the highlighting ... yes, it should, considering multiple hands ... thinking aloud here). Then the dragged morph wouldn't have to be translucent, but other styles may be used (like the attached screenshot from Galeon).
Ah, the hand could just take on a different appearance if dropping is available! It could be highlighted, or it could be some other shape. That would seem a best of both worlds.
-Lex