Macerator

[Valvebloke]

... The only other tricky looking job is cutting into a cast iron soil stack. A decent quality Bosch or Starrett holesaw run slowly with plenty of water as lube ought to do the job ...

This has probably already crossed your mind, but consider how you're going to stop the cut-out disc from falling down the inside of the soil stack. It can be surprising how a small, dense, rigid, insoluble object can block a pipe. Since you'll be cutting into a curved surface the disc will eventually be held in place just by two small uncut sections. You might finish those off by hand having poked a coathanger hook through the pilot hole. Or a longer than usual pilot drill might be enough to do the job.

VB