I work as an industrial chemist & handle similar situations, so can tell you that the muriatic acid approach is worth trying. Even tho it will, of course, react with & corrode the iron pipe, it will react with the cement in the concrete first. Wear safety goggles & gloves & expect some fizzing bubbling as soon as you pour it in. Try rodding it as you go & don't pout too much acid in at once - say 1/2 pint at a time. If there's a lot of concrete in there the rocks/sand may be helping with the clog, but if not I think you'll have some luck. If you want to get really scientific try monitoring the pH of the water in the pipe & keeping it as close to 6 (or higher) as you can before adding more acid.