If the radiation is just scattered i.e. it has the same (or very similar) energy as the incident radiation it will NOT be absorbed by the wax for exactly the same reason as the original radiation (x rays) was not.
On thing wax will do is to moderate neutrons that may be generated by high energy x rays ca > 8-10MeV). I am not sure what energy is currently used in IMRT of oral cancer... and if neutrons are even an issue here.

Re tongue: as long as your RO (and the person who actually calculated your map) knows this you should be ok. (you are moving an anatomical structure that is in the radiation field and that now gets a different dosage).

M