International Auxiliary Languages are very, very ambitious, and probably bound to fail on a large scale. However, it seems plausible to me that there might be some corners of the world where an auxlang might be used to facilitate communication between more localized groups, much like English is used in many parts of Europe (and really, the world) for this purpose. But does this actually occur anywhere?
The line between "conlang" and "supradialectal standard" can get incredibly fuzzy. You mention Rumantsch Grischun and Modern Hebrew yourself, something along similar lines, but much more successful in terms of number of speakers is Bahasa Indonesia. Any number of other supradialectal standards depending on how strict you are with drawing the line may fall under this category.
In addition to supradialectal standards, another thing that may or may not be considered a conlang is controlled languages, such as ASD STE-100 Simplified Technical English which see some use in various technical documents (but aren't really spoken).
I know of one very localized example myself:
Rumantsch Grischun is a constructed variety of the Romansh language which tries to unify the dialect continuum. It is the variety of the language used in official texts and also in some schools and media, but the population in general is rather unhappy about this status quo and prefers using either their own dialects, or Swiss German if that fails.
In addition, some consider Modern Hebrew a conlang, and it does serve as a lingua franca between various Jewish populations. But this is definitely a less prototypical example.