Believe it or not, we have done that multiple times at FSG and never caught a team really cheating. You can say whatever you want. The noise test was just not suitable as it was for keeping the noise of the singles down on track - end of story.
You can run an off-the-shelf muffler. Just use a 4cyl. engine. You think your will be less competitive then? Oh, so you have to do the extra work. I don't see why it is the task of the RC to make it easy to run a single.