This is the state of the road in Mustang as of April 2013. The road to Lo Manthang splits away from the walking trail at Tsaile / Chele.
From Chele (Tsaile) to Samar is bad road. Most of the walking trail is non-bikeable at all. The new motor road can’t do two people on a bike as a bike can hardly pass the uphill.
You can get a jeep/bus upto Chuksang from Jomsom. From there, you’ll have to walk to Bhenya (this is between Samar and Syangbochen). From Bhenya, you can get a jeep to Lomanthang directly.
Thanks to Tashi Bista of Friends of Mustang for the information.