Is it religiously permissible for a woman to perform istinja' with the water she uses for ghusl to purify herself from menstruation for the purpose of conception, and is there any prohibition in that?

We do not have anything that scientifically confirms the validity of this treatment, nor do we have specialists in dream interpretation. We are not aware of any religious prohibition regarding this matter. However, it is best to seek out proven treatments, the most important of which is supplication (Du'a), and to seek treatment from specialists.