Java: slower.
C++: Harder to edit.
Neither can be copied in any way from the BYOND code.
The result is that you have to create a game engine then build the game on top of that.
Java won't work as well, as they specifically state that BYOND, although not as fast as a compiled language, uses some highly optimized assembly, especially for list operations, although it might be slightly easier to write.
C++ is certainly an option, but I think that I could just as easily write it in C. How? Because the first C++ compiler, I think I read, simply converted C++ code to ugly C.
Function pointers.
Structs as vtables.
In general, you could remake it in almost any language, but BYOND is especially suited to the task, and has access to the existing code base.
What you want, then, is a custom non-BYOND master server lobby not accessable through BYOND, and maybe an external authentication system, to split it from the BYOND version. However, that means that only people who go on the Bay12 forums would know that it exists until somebody leaked a link.
And:
"c++ is byond code's older brother"
No, BYOND is a young boy whose role model is C++. C++ couldn't care less, as it has many fans, and is too busy working in the commercial game industry to care about every little programming language that took inspiration from it.
In fact, BYOND uses a primarily script language, compiled to bytecode(like java) but on a custom interpreter optimized for games, on a platform optimized for games, and with language features optimized for games in a way that C++ never could(without starting to look "a bit" uglier than BYOND's code ever could)