Modders would need to provide their own code but there is no way for us to make this a practical option. Modding HL2/Source is a very unique setup as the game source code is provided and mods are distributed as their own individual entity; the only unifying factor between them is shared content through the use of required games (like HL2) and engine code (SDK bases). Handing out the source code for BM and letting people create their own code-based mods could turn into a real nightmare for everyone as every BM mini-mod that has code changes would replace the DLL files. Ultimately this would lead to incompatibilities with other mini-mods and create a big headache.
In theory, you should be able to reference other directories in /sourcemods as gameinfo.txt search paths to use their content in a different, custom mod. However any custom code would require new DLLs for that mod, and would override/take priority over BM’s DLLs.
Ultimately, everyone should have a good range of freedom to edit what they please within the mod, but code is really out of the question.
EDIT: This doesn’t apply to any multiplayer that may come along later; mind you - most servers will probably be running as pure servers; so all content will have to match to avoid cheating (via material ‘hacks’, etc)