@Tanner write API could help with that, but until then, here's the high-level implementation/workaround:
FTPP (form-to-post plugin) establishes new POST route at /feedback
When that route is called, take req.body.text (where text is the id in the corresponding form) and call Topic.post as a pre-registered user.
That's pretty much it. There's more to expand on (such as spam prevention/throttling/auth/security), but yeah.
Don't ask how I came across this one, it's a strange one, to reproduce,
This only happens in Firefox as far as I can tell,
Open a topic via ajax from a category, close the tab with ctrl + w or with the X on the tab, press Ctrl Shift & T to reopen that tab. Hold the back button on the browser and click the first link that takes you back to the category where you came from.
You get this:
However the URL is that of thr category. 👍
It actually looks quite nice, but should probably be an option if people ask, and not a bug. 😆 (Would tie in nicely with the related posts issue that's on Github)