Reloadable Lua "Modules"

[Irayo <irayo.lt@...>]

I'm designing a system in which various "modules" work together, 
handling different events, in order to provide an overall result.  I 
know this is extremely vague, so let me explain some more before I get 
to my question.
Let's assume I had a robot, for example.  Under my system, one module 
would handle the hand movement, one would handle foot movement (and 
coordinate walking), and one might handle speech.  Unfortunately, 
there's a couple of catches.
Modules should only be able to access certain members of other modules.  
I've got about ten people or so who have expressed interest in scripting 
a module, but I don't fully trust them all to not execute functions that 
might destroy the bot (or even just an accident, like unloading all of 
the other modules).  I don't want function collisions, either, but I 
don't want to lay down a huge system of rules or naming conventions for 
its script writers to prevent multiple modules from having functions of 
the same name.  However, certain modules need to be able to communicate 
with certain other modules (for example, hand-eye coordination, or 
remembering what the bot was doing).  I'll also have some modules that 
will be only libraries; that is, they'll have some basic functions that 
all the other modules should be able to call.  Also, each module needs 
to be loadable or unloadable at will (unless of course other modules are 
depending on it at the time).  If I need to fix a bug in the eye system, 
I don't want to have to restart the entire bot, which will then cause it 
to forget everything in its other modules--and if the eye system has an 
error, I'd like the bot to continue with its other systems, ignoring the 
fact that its eyes are no longer open, until I tell it to reload the eye 
script.
At first I was going to have each module be a separate Lua state.  
However, I couldn't find a good way to allow separate Lua states to call 
each others' functions.  It seems the only viable option is to use a 
single Lua state, but then I do not know how you could "unload" a 
certain section of that state without completely resetting the state.  
It's possible that I could store all of the functions and variables of 
that module in a table, which could then be nilled when that module was 
unloaded, but I don't believe that tables can have private members 
(unless I'm mistaken), so that still wouldn't solve all of my problems.
So, what's the best way to implement multiple reloadable scripts and 
make sure that they don't have member collisions, and yet have them 
still interact with each other on a limited level?
I'm completely clueless, and I'm terribly sorry if I've just wasted 
everyone's time anyway....
--
Irayo