There are some new cloud HR tools that may do this for you.
I have worked on systems where internal staff were added via API to a WS/LP and were managed by HR for this purpose. Perfectly fine to do. I've spoke to people who have used marketing automation to manage HR processes for internal needs and for external candidates. You just need to make sure the process is setup.
I doubt that you will find a tool that will not require some tweaking of emails. You could switch to Email 2.0 and perhaps reduce the problems HR could have.