I don’t know, cc.RenderTexture looks like some cocos2d-js left-over actually
I just shared my implementation of somewhat similar techinque we used for scene post-processing in a big html5-only project.
Its been sometime since you post but I will just try answer it anyway…
Instead of directly appending it as a Sprite Frame, create it with proper size might help
var texture = rt.getSprite().getTexture();
var size = texture .getContentSize();
var sf = new cc.SpriteFrame(texture,cc.rect(0,0,size.width,size.height),false,cc.p(0,0),cc.size(size.width,size.height));