Re: Reject SP Resource Request via Webservice
Short answer is "No", but as usual there's always a way if you're willing to go down the path of unsupported REST APIs.
Go to the manage allocation page (make sure to use the new Gantt view), and reject a position while monitoring HTTP requests sent by your browser (using either your browser's dev tools or Fiddler). You'll see that it'll do a POST /rest2/position/reject with a JSon payload including the comments and the position info. It'll pass all the position details, but I don't think you need to pass the full details for it to work - I guess you'll need to experiment and see what works for you.
As this is not an officially supported API, it's up to you to do the reverse engineering if you really decide to use this.