This is not possible atm. The current loading mechanism relies on the compiler for loading extensions that appear in the source code automatically, which is convenient, but not always appropriate.
Is there a way to achieve this without such a specific compiler feature? It seems necessary to be able to write to a variable at compile-time and then also have the ability to use it at runtime, which would be cool, and it would probably be useful for other things too, but I don't think it's possible now.
This can be worked around by writing to a file at compile-time and then reading it at runtime, but will it ever be possible to use a global variable for this?