Area53,
Thanks, my wife is really good with her sewing machine.
I plan to carry a dry bag on one side and a backpack on the other side. Yes, it's a lot of gear but we'll be living on the road for half year so are splurging on a few items. Since we'll be living primarily in a tent and camping most nights, I'm planning to bring a huge tent, a Marmot Limelight 3. I expect it will wear out before the trip is over and will reevaluate the shelter then.
In the photo below, I have the backpack pretty full but on the trip I'm planning to keep it nearly empty so I can use it as a day pack for hiking and grocery shopping. I seem to have more trouble with neck pain when I bike with a backpack so will try and avoid that.

Smo,
My strap ends aren't long enough to be a problem. What little extra strap there is can be tucked in between the bag and the sling.

My next project will be to make a handlebar sling with detachable handlebar bag. I've gathered the parts for the sling but the part I'm still working out is a design of a bag that can be easily unsnapped from the sling and carried as a hip belt, messenger bag, backpack or ? I'm wanting something very small, just big enough to hold my wallet, passport, sunglasses, snacks, etc and that I'll unsnap from the bike and wear into shops or other times that I leave my bike. Either something I can buy or make. Any suggestions for this would be welcome.

Here is an idea for a light weight pannier system. It is basically a sling that a dry bag slides in to. I know it's not a normal bike packing system but we are planning a 6-8 month trip, primarily on pavement but with some trail riding. We'll be carrying a lot of gear, about 25-30 LB per bike plus food and water. With that much gear, I'd like to keep the weight as low on the bike as possible.

The sling weighs 160 grams and the 25 liter dry bag weighs 190 grams.  The rack is a Topeak Explorer and weighs 625 grams.

The blue fabric is from some heavy duty rain pants and the yellow material is from a school crossing vest, both purchased from a thrift shop. The webbing and buckles are .75" wide except the center belt is 1" wide. The plastic tubing shown is slid into a pocket where the blue and yellow fabric meet. It allows the blue fabric to be stretched tight and it helps to keep the dry bag flat when the middle webbing is tightened. There is a loop of .5" webbing that goes around the tube end at the bottom of the rack. In line with this loop is a buck that connects the sling to the top of the rack. This is the main attachment to the bike. There are also velcro in the top corners that hold the fabric in place but carry no weight. The blue material was used near the wheel because it will be easier to clean, the yellow material was used on the outside for reduced weight. The silly pattern that was on the vest is just a bonus. 

The webbing is sewn along the edges and the ends buckle together to form an entire loop, effectively hanging the dry bag from the rack.
It only takes 30 seconds to attach the sling to the bike and another 30 seconds to install or remove the dry bag. To remove the dry bag, You unsnap the middle buckle and unsnap one top  corner buckle. My plan is to leave the sling on the bike at night so I'll only need to pull out the dry bag.

I am considering making an even lighter version but would like to do quite a bit of riding with it first to see what improvements are needed. For the improved version, I'd like to use a strong and light material for the sling so that it carries the weight of the dry bag and would not need to be reinforced with webbing. I'd be pleased if I can get the sling down to 100 grams. My current dry bags weigh 190 grams each. Ortielb, Exped, and others make similar sized bags that only weigh 80-100 grams.  I'd like to go to a rackless system but I also have a desire to carry the weight low so not sure how to accomplish both.