I agree that these warning should be fixed anyway. I'd give them a go without any hesitation, but I'd like to know whether Drupal is fully UTF-8-aware and whether UTF-8 support is a requirement for the modules - i.e., whether my work would be useful in the end.
Adding
php_value mbstring.func_overload 7 php_value mbstring.internal_encoding UTF-8
affects everything just for CiviCRM's use, and if it's known that Drupal would break (in some non-obvious way, perhaps), or there's a popular module that could stop working when we're overloading all of the string manipulating functions under its feet then I'd rather concentrate on other solutions to my problem...
Please excuse me if my questions are simple and/or obvious, I'm very new to Drupal developement and even Drupal in general.
We will see, if someone jumps up and informs us that this was already tried. Nothing is supposed to break of course if you use mbstring, but I never tried it myself. Goba