Is the concert during a time in which the Weirs Channel Bridge is closed to 4 wheeled vehicles and are you driving a 4 wheeled vehicle?
When I have to go from Meredith to Glendale and the bridge is closed can only think of a few options.
1. Pick up Route 106 and head to Laconia but remember that's probably the most popular route.
2. Head down 104 and pick up Meredith Center Rd and follow that into Lakeport/Laconia. You come out near Irwin Marine and can take it from there.
3. Head out 104 to Route 93 South to exit 20, then come in as if you are going to Meadowbrook.
All three options take a while but they will get you around the bridge.
Of course you can always drive south to Alton and come up the other side of the lake
It is a very good question and I'd love to hear other alternatives because I do have to go around that bridge as well during bike week!