Miklós Galicz posted a short article on how he managed to override Laravel's str_slug helper function.

Long story short, until this is resolved one way or another... A really obscure but powerful tool can be a temporary solution for this. It's called Helper Overloading. Laravel's helpers are created in a way that checks if the method already exists. ... This is really great, the only thing remaining is to actually add our own method before Laravel creates it own version.