I contemplated making a dual or duo servo brake for the front, because it appears to be a leading/trailing shoe. However, upon rereading the rules they specifically required the bike to have a trailing shoe. I saw that as a stopper.
The duo servo shoe has a primary and an secondary shoe. Those shoes are both linked together (where the pivot would otherwise be). The leading shoe has a cam and the trailing shoe has a solid abutment to hit so it does not move. That abutment could be the constant radius cam or a lump welded to the backing plate.
The real trick is that when you move the cam both shoes are forced by one the single cam to move in the direction of the rotating wheel, which they will gladly do. As the leading shoe (primary shoe) grabs the drum, the drum it is jamming itself into the into the trailing shoe (secondary shoe.) Because the back end of the secondary shoe cannot move due to the constant radius cam or the abutment, all that force causes the secondary shoe to be pushed hard into the drum. Because the back end of the leading shoe is connected to the leading edge of the secondary shoe it is pushing hard on the leading edge of the secondary or trailing shoe. Exactly what you want!
Just for clarity, the blue colored abutment at the top would be attached to the backing plate or it would be a constant radius cam.
The cam, which is not shown, would be pushing the shoe where the small red "block" arrow shows. The cam is shown at the top of the drum here, but it could be rotated to the bottom.
At the bottom under the link is another "block" arrow showing the transfer of force from the primary shoe to the secondary shoe. The leading edge of the secondary shoe is the one that is being forced into the drum by the link.
The one sided constant radius cam from a Triumph is shown here:
The cam lobe is on the bottom in the photo and the other lobe does not exist, but is a constant radius so the shoe that is resting on this side will not actuate even as the cam spindle turns. This way it works one shoe only.
I have read that these brakes are better than twin leading shoes in terms of braking. I have also read the the primary shoe does not generate as much force as the secondary shoe. This would lead me to believe that a twin leading shoe might have a bit of an edge. I don't think you will have to worry about spragging with this brake design any more than on twin or 4 leading shoe brakes, but it pays to be careful.
David