You dont even have to use the PVC flange. Get a cast iron or bronze DWV offset. All you will have to do is chip a small shallow slot a couple inches deep around the pipe with a chisel to accommodate the new flange, caulk it in with oakum, and cap the oakum with propoxy20 or JBweld or something similar. This is the easiest way to avoid chipping out the entire slab.
This is the problem, stubbing your toe is where the homeowner gets stuck. Years of trying to fix these problems is where the tradesman gets the "fixes". They dont just come to you in a coveted pamphlet the day you get your license. You spend years and years coming up with better and more simple solutions to more complex problems and yet the homeowner always thinks you are a gaffer.
I hate to enter the homeowner vs. contractor debate but this thread shows clearly that alt.home.repair, or at least the mindset its readers have posted here, is a scary place. I would venture to guess that without google half of the posters to a plumbing thread dont even know what "oakum" _is_ as they have never seen it when bopping through the isles at home crapo or howes. How about a caulking iron? spooning iron? inside chisel? outside chisel? joint runner? resilient gasket?
I have no problems with homeowners doing their own work, in fact I encourage it. But you get what you get and dont bitch about it when what you get is substandard.