It's hard to argue against C (or other procedure languages like Pascal, Modula-2, Ada) and assembly for embedded. There is a large history of success with those languages. Generally, you want to remove the risk of the unknown. Trying to use anything other than C or assembly, in my opinion, is an unknown. Having said that, there's nothing wrong with a mixed model where you use one of the Schemes that go to C, or Python or Lua or JavaScript as a scripting language.
What you want is the ability to quickly and easily go to C when you have to.
If you convince the team to go with something that is unproven to them, the project is your cookie. If it crumbles, it'll likely be seen as your fault.
