That function can be called when the register form has been submitted. It will return true or false. Forget about the transient stuff, that is just WordPres caching stuff.
You don't need an API key like with Akismet. You would only need it if you want to add or remove entries from the SFS database. It really is much simpler. Ofcourse you might want to have a checkbox in the settings. But still, in an afternoon you might be able to finish this :)
