To start off with, forefoot varus is very rare and it is important clinically to distinguish between forefoot varus and supinatus.
However, if a true forefoot varus is present the traditional approach is to use a forefoot varus or medial posting to bring the ground up to the foot, so that the rearfoot does not have to pronate to bring the medial side of the forefoot to the ground. This is the traditional approach to managing forefoot varus.
If you had a forefoot varus post to a rigid plastic orthotic, it will tilt the plastic plate to affect the way that the rearfoot will not need to pronate to bring the medial side of the forefoot to the ground. Place the foot on top of this rigid plastic orthotic and theoretically, the rearfoot will not need to pronate.
HOWEVER, consider the effect of a forefoot varus post on a flexible/semi-rigid orthotic. As soon as the foot is on top of the orthotic, the orthotic will be resting on the shank of the shoe. There is no way that the forefoot varus post can affect the rearfoot. As the orthotic is not rigid, the forefoot varus post’s effect will not be passed to the rearfoot. All that the forefoot varus post will do in this situation is dorsiflex the first ray. If it does get the first ray to its end range of motion, it will then supinated the midfoot joints. The only way that it can affect rearfoot pronation if it moves the first ray to end range of motion and then supinates the midfoot joints to end range of motion. Surely moving these joints to end range of motion to affect rearfoot pronation has got to be a bad thing.
Is forefoot varus posting potentially injurious?
See also: this and this