After a quick test using code from CameraFramebufferTest in a new project I found that it should work for you, but performance testing would need to confirm there’s any benefit (there should be a large benefit ideally due to much reduced pixel count, except for the final render of the sprite with the frame buffer texture).
I needed to set design resolution to same as frame buffer or it won’t be pixel perfect since it will texture filter out pixels or stretch them due to UV mapping.
glview->setDesignResolutionSize(420, 240, ResolutionPolicy::SHOW_ALL);
auto fboSize = iSize(420,240);
auto fbo = cocos2d::experimental::FrameBuffer::create(1, fboSize.w, fboSize.h);
also in a quick test I needed to flip the sprite used for rendering the frame buffer to screen
auto sprite = Sprite::createWithTexture(fbo->getRenderTarget()->getTexture());
sprite->setFlippedY(true);