Looking at the engine source and trying to follow what happens with the BlueprintAutocast metadata this seems to be the flow:
All functions with the BlueprintAutocast metadata are added to the FAutocastFunctionMap singelton. This map is refreshed for example when you HotReload from the editor (FAutocastFunctionMap::OnProjectHotReload) or a module changes (FAutocastFunctionMap::OnModulesChanged). The FAutocastFunctionMap::InnerMap maps strings to TWeakObjectPtr<UFunction>. The string keys contain the type information of the first input pin and the return property.
So now we have this map available where is it actually used?
The AutocastFunctionMap is searched for an appropriate entry via FAutocastFunctionMap::Find using the input and return pin type information. The search is done in UEdGraphSchema_K2::SearchForAutocastFunction. It is used to determine whether a connection is possible (UEdGraphSchema_K2::CanCreateConnection) and in UEdGraphSchema_K2::CreateAutomaticConversionNodeAndConnections. This function is called when you try to connect a wire for example from a ‘float’ type output pin to a ‘string’ type input pin. Thanks to the BlueprintAutocast function/system, the function call to UKismetStringLibrary::Conv_FloatToString will be added automagically 
In your case this means when you drag a wire from a UMyObject* to a string input, your Conv_MissionStringBindingValue function should be automatically created for you.
Is this what you were expecting to happen with the BlueprintAutocast metadata or were you looking for something different?