Shoutbox

You can use Answering Machine Plus! for auto replying and you can define a secret word for it to send /assist command back to you.

However, as far as I know when Remote Assistance is first started, it is enabled for only Viewing the Screen. To gain control over that PC, the person who asks for help should allow you to get control and Plus! can't do this normally and I think it mustn't, because it is not an add-on to do such things actually.