I know the rule and I'd tell anyone the same thing you did, don't try to fix the sidewall.
BUT… I worked at a gas station when I was in high school for several years, and I patched a LOT of sidewalls. It's touchy, you have to use common sense. Small holes that haven't torn the tire and stuff can be done if you have good chemicals and patches to work with. Bigger rips and other stuff that may have damaged the structure, no way… avoid it unless it's an off-road farm use thing or something similar. But done right, the patches will stay put and hold air. for punctures, plugs are JUST FINE. I must have plugged 1000 tires with them and never had a single one come back leaking. There's absolutely no reason to not use plugs. I even use those off the shelf kits now on my own tires and they work great.
Oh yea, and it's a lot easier to do the job with the tire aired up all the way, just hold your hand over it so you don't end up with glue getting blown into your face.
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