I ran through a bunch of variations and came up with something like the Exped Dry Pack and a smallish fanny pack with water bottle holsters. My idea was to get the weight on the fanny pack and only carry the lightest but bulkiest items in the rucksack. This idea was oreinted more towards helping someone whould couldn't tolerate much weight on their shoulders. As an offshoot, I was trying to make a silk purse from a pack that was just too small and needed more room for water bottles and essentials.
It is barely worth the effort, as the extra fabric and hardware just increases the weight. My conclusion was that a very light pack that could still transfer weight to the hip belt was a better option.
I think there is a tinkerer's appeal to many ultralighters in having a micro-kit that can support a hiker for at least two or three day trips. The first thing that happens is that it becomes a summer-only proposition, with micro-light sleeping bag, minimal pad, etc.
I'm still trying to go there myself, working on getting the bulk down. I mentioned my 5oz kitchen in another post, which is born from this very idea.
So far I'm using a run of the mill UL pack, small cook kit, chemical water treatment, poncho/cape shelter and conceding to a RidgeRest pad and a 32F sleeping bag. Clothes can vary with the season. It gives me as light as compact kit as I can expect and still have some reasonable safety and comfort.
If you want something for day hiking away from your main pack, there are a number of stuff-sack-with-shoulder-strap designs that will work and offer more capacity and comfort than a fanny pack at only an ounce or two more that the lightest fanny packs. The Etowah Outfitters "Town Bag" is agood example:
Integral Designs makes a larger version. See: http://www.integraldesigns.com/product_detail.cfm?id=770&CFID=7378347&CFTOKEN=43005992&mainproducttypeid=1
I am using one of the Etowah bags as my ditty bag and emergency bug-out bag. It only weighs 2.1oz and can carry rain gear, essentials, food and water if I had to drop my main pack and head for home in a hurry or was injured, or just wanted to day hike away from camp.