I'm not sure about the dll, but you could just write your own application in Autohotkey and package it along with your C++.
The communication takes place over a hidden window with an edit control and a button. You use one application to set text in the edit box, and then to click the submit button. The other application owns the window can process whatever is put into the edit control - as if you were passing a variable. Basically, that's it.
Check out this thread where I've explained it in more detail: How to send a command to a running application via commandline
Now, that's not quite what you wanted, but the effect is the same, and you already know all the api.