all great ideas!
personally I'd like to see a way to place bundled rope/cloth/chain/etc. as a decoration.
some items should just be placeable on their own, since they have no other use without certain applicating devices (paint, dye, silk)
for a lot of the stuff that has a use (food, potions, crystal heart, etc.) why not have a recipe at the heavy crafting bench that turns it into a placeable block? they could be collected as normal and turn back into the regular forms (like potions on shelves in the dungeon, or generated crystal hearts). This could be annoying since you may have to run back and forth re-crafting stuff if you pick it up, but it makes the most sense to me.
for certain items I could see various wands (similar to living wood and bone wands) working pretty well. Some could be rare drops (fairy wand is a rare drop from fairy's, ectoplasm wand is a rare drop from ectoplasm, etc.). I could see the dryad selling a mushroom wand for the currently non-placeable mushrooms. Or, you could have something like how you can craft stone and crystals into crystal blocks, where you craft components into the block you want instead of them just being immediately placeable.