How to add C functions to a Lua module within the require process?

[Ignacio Burgueño <ignaciob@...>]

Hi everyone.

I'm writing a C library which has submodules written in Lua. So, I have 
Foo (a C library), Bar (a Lua module) and Baz (another Lua module).
require "Foo"
require "Foo.Bar"
require "Foo.Baz"

So far, so good. The problem I have is that I need to add a C function 
to Foo.Bar and I can't find a good way to do that.
I tried the following:
1) in luaopen_Foo, I do:
   luaL_Reg functions[] = {
   { "func", a_c_function },
       { 0 }
   };
   
   luaL_register(L, "Foo.Bar", function);
   lua_dofile(L, "/my/path/to/Bar.lua");
   
While that works, I'm forcing the load of "Foo.Bar" even when it won't 
be used.
2) using package.preload, placing a function to handle "Foo.Bar". Here I 
have two issues. One, since /my/path/to/Bar.lua could change (for 
instance, now I'm deploying with
LuaRocks 1.0 but that could change in the future) I should issue another 
'require' (but I'll call myself again).
I've considered adding Bar.lua as a resource to the library but I'd like 
to avoid that (I don't want to recompile Foo.dll for each change needed 
to Bar.lua)
Basically, something like "package.postload" would be what I need. Some 
hook to do stuff *after* a module has been required.
Is this possible or am I going the wrong way?

Regards,
Ignacio Burgueño