Many thanks to Milo Dolezal for the invaluable advice.

Bring the cleanout to grade and install a recessed clean out cover (see image) in place of s regular one. Good luck, Tom

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Thanks for sharing this experience. I am wanting to do something similar here in Australia and your pictures are inspiring.

Hitting the mainline from the newly installed 1 way c/o will do unless you get mainline congestion BEFORE that c/o. IF that is the case then installation of a Kelly fitting (2 way) will be needed.

However, running a cable TOWARDS a home can result in unwanted results if the tech is not experienced enough i.e. broken toilets and/or broken traps. These unwanted results are just some of the reasons there are c/o's installed in the backyard or side-yard that tie in and run the main, through the house, to the street.

Techs will usually put a "whip" on the end of their cable to eliminate and cut through congestion. This "whip" can and usually will cause the cable to follow turns and/or bends that are meant to lead AWAY from fixtures, not towards them as running a cable backwards can and will cause the it to do.

The best idea would be to have a mainline c/o installed in the backyard or side-yard that runs to the main, ties in, then runs to the street. Either way you would have to get rid of the clay wye out front as the cable would probably want to follow it once it got to it and would then break through the plug. This would obviously cause a larger problem.

So, you have more than a few options all of which are time consuming and require a fair amount of work and material. On a lighter note, the tech could just run cable down a 3" roof vent and clear any congestion all the same with no alteration of your mainline.