I'm not too sure how one could implement hiding of upvalues at the
language level. (At the implementation level, it's obvious. Just skip
the phase that looks for them.)
This is not so obvious because Lua highly depends on this; the "phrase" that looks for it is exactly the one that lookups variables in the environment using its "__index" meta-entry, which is where the environment is already stated: so the first level of lookup would be required (otherwise the function itself not would have itself access its own local variables) but you want to avoid the recursion of the lookup to the next level to look for upvalues.
Note that this recursion is a trailing recursion (so Lua optimizes it natively as a loop: the "phrase" you want to hide would be a statement within that loop, and you want it to be used only on a specific loop number to break that loop by returning early a "nil" value so that an "undefined variable error" can be stated). The difficulty is that there's no loop number which is accessible. So all I see you can do is to set the "__index" meta entry specifically to your need.
