Forty Rod,
One suggestion, depending on what you are using the concho belt for now, is to simply wear two belts. I've seen quite a few photos of individuals wearing two, one for their sidearm, the other for a knife, sword, or another sidearm. Individuals during that time were very resourceful, and those on frontier especially were not held to strict dress code standards.
Is the sheath of traditional wester design, as in the belt loop sits directly behind the handle, or is it of more of an older design where the loop is actually a slot off to the side of the knife? Being from Dixie and designed for a green river knife, I bet it's the latter. In that case, there really isn't much you can do other than wear it just tucked into the belt, which was quite common.
I know this really doesn't help a whole lot but it's one of those tough circumstances.
Laff-a-lot Louie