[mixedCase]

That sounds like it belongs in the top 10% of the worst (programming-related) things I could wish upon another programmer: Having to maintain projects where you have to read the build system's configuration and keep it in mental context just to figure out if an array access is mathematically correct or not.

I would imagine this support to be stomachable if it was active at all times but using a visually different syntax for 0-based and 1-based indexing, maybe using something like the guillemet for the latter (like this: «1») that is wild enough to draw the user's attention when reading the code, and not common/prominent enough across keyboard layouts to make most users think twice before using it when writing code unless it makes sense for the domain.

[lambda_obrien]

It's hard enough to understand a codebase, but if a language was like Haskell for instance (you can enable certain extensions in Haskell with a flag at the top of a file), you could have a line at the top of a file defining that this file is 0 based, while maybe the default is 1 based for the language. It doesn't seem like that much more than you already have to consider in many languages.

[mixedCase]

Funny you put it like that, because the fragmentation regarding language extensions is frequently discussed as one of the biggest issues affecting real world Haskell.

[tome]

It's mostly talked about by people that don't use Haskell! In practice it's not really a problem.