Is it permissible to masturbate with the intention of performing ghusl before going to a place where water is not available, in order to avoid a wet dream that would necessitate ghusl?

Masturbation is not permissible, as it is forbidden by Islamic law. If it occurs before the end of the month, an expiation for breaking an oath (kaffarat yameen) is required, along with repentance for this sin. As for a wet dream, it is not a voluntary act, and there is no sin in it. Fear of a wet dream does not justify masturbation. If you experience a wet dream, ritual bathing (ghusl (definition)) is required if water is available; otherwise, dry ablution (tayammum (definition)) is obligatory.