It's actually perfectly possible to use FB credits/Chrome Web Store payments/whatever in Construct 2 already, as you say you just need to follow each site's developer instructions to integrate it with the server side scripts (which is the only secure way to do it), then make a custom plugin with the SDK to talk to the server and see what's been purchased. Obviously if this was integrated with the built-in plugins it would be a bit easier, but if you're going as far as setting up the server-side scripts then using the SDK isn't that big a hurdle either. Plus on top of that your own plugin means you can add your own security system and custom functionality, which means it's probably harder to hack than having a one-size-fits-all official plugin.
So I think there are people sitting around waiting for us to do something to enable IAP, which actually just means us making some tweaks to plugins that makes it marginally easier. If you're desperate for IAP there's nothing stopping you going for it now, but I guess you'd be the first.