I would probably allow it. Like you said, C# is not exactly mainstream in Godot yet, but I was hoping it was familiar enough as the second main supported language that users might try it out for this jam, or at least be inspired by the entries in the jam. You are totally welcome to use any .NET language you like, I just can’t help out much because I’m only familiar with C# myself.
If you choose to join, best of luck <3