After I find the soil pipe, which will likely be cast, can I knock a hole in it and use a saddle, and cover it all with concrete?

IT's not that simple Glenn,
A fitting will have to be cut into the main and a vent run for the toilet. Since you're going to hafta vent it anyway why not tie a lavatory into the toilet drain and vent out the lavatory. That way you'll vent the toilet and have a 1/2 bath instead of just a toilet. Good luck, Tom