Are historic Steam playtime stats available?

IF Steam is collecting such a detailed statistic, then I haven't seen any documentation on that and it most definitely isn't available via API.

Although, they do collect quite a range of data across all users, presented e.g. via here or here

AFAIK, detailed statistics for single games are available to the games developer/publisher on steam, but not the users.

The easiest way to build your script that I can think of would be to query your Steam community profile for the currently played game, this is queryable in various formats such as JSON or XML. But this requires you to set your profile to public.