I don't think Facebook offers API's for Whatsapp, so that other developers can use the Whatsapp messaging engine to make nice apps like you describe.
But it should be possibe to write such an app using SMS-messages. If there is none, and you need it, have it written or write it yourself for Android, iOS or both.
SMS on a PC seems a dead end.
I'm looking for an app for iPhone or some kind of software for a PC that will let you make, lets say 10, different text's that should be sent out to a group of designated people.
If the 10 texts could be copied, so they can be used at a later time again, that would be fantastic!
Hoping you can help me out.