Judging from the fact that woodcutters automatically pick up an axe, it can be assumed that enabling professions allows automatic changes in equipment.
So, why not allow hauling to have the dwarf equip a back backpack or sack? Dwarves (planters, specifically) already carry sacks and use individual items from them as necessary.
Also, since stockpiles can be limited in terms of what they're allowed to take, it might be easier to approach mass-hauling issues from a stockpile-centric approach: instead of looking for specific items that need hauling, a simple counter designates how many items of each type are needed for transportation. Any time that counter has a value greater than 0, the appropriate hauler dwarves first find a stockpile and see what goods it accepts, then scan the goods in need of hauling for those that go in that stockpile.
As far as bags, it seems that bins already allow differing numbers of objects based on size (I get about ten bars per bin, but easily twice that many rings, idols, bracelets, and totems), so the same sort of formula might be used to determine how many items a dwarf can haul with his bag. Of course for things larger than the bag, like statues, they can always carry one around in their arms to wherever it needs to go.