To be pedantic, it wouldn't be "application/csharp". By the time it's compiled to MSIL bytecode it doesn't matter if it was C#, VB, F# or whatever. "application/MSIL" is what I'd say.
I can imagine it, and I guess my browsers already have three VM/Runtimes in them (JavaScript, Flash, Silverlight) but it feels so inelegantly top-heavy to have lots more.