""Zool - yes, the sign is carved only on one side. The way it has cupped this is the outer radius of the cup. That's the reason I'm thinking of relieving the back by cutting a few slits (3mm wide by possibly 6mm deep) along the back face to relieve stress. There again I'm not sure if this would introduce a weakness along these lines as the timber expands due to weather conditions. If I do this I would still use the back braces - my thinking is that this would prevent it splitting along these lines.""
As the board is not a structural/weight carrying member, probably cutting reliefs in it will not cause weakness problem. Cuts that are 3- to 5-mm [0.10- to 0.25-inch] in a board "25mm thk x 200mm wide and 900mm long (grain running along the 900mm)" shouldn't be cause for concern.
The back braces will help, but as Louis indicated, make slots for the screws in the braces rather than holes so the screws will not be fixed in the braces and then split the braces.
I think that overall we are all suggesting to analyze the material you are using and anticipate movement, and take actions to minimize the adverse effects of that movement.
If all else fails, possibly pickle the sign in a good single malt. Maybe that will keep it calm for an extended period of time. If you don't want to use the single malt on the sign, then you and the sign's owner might sit down with the bottle. Soon the sign will not matter.