I don't think a bloated virtual-category bag mod, or an item-set addon, are what the OP is looking for. It sounds like he's already got his bags and his item swapping working as desired; he just wants something to move specific items into specific bag slots.
I know Necrosis used to do this for soul shards back before the introduction of shard bags, but I don't know of any standalone addons that do it for arbitrary items.
Also, wrong forum. Moving.
That's exactly what I'm looking. Thanks for the answers, haven't been able to check back before now. And a little bump. :-D