I haven't made a Guinness clone and I am not setup for nitrogen but I have kind of simulated the cascade you get with canned Guinness by using a syringe. I use the kind you get to orally administer medicine to babys/small children.
I leave at least an inch of space at the top of the glass, suck up 2 ml of stout into the syringe, hold the tip of the syringe just over the surface of the stout in the glass, blast the contents of the syringe into the glass. If you plan to try this I recommend doing it over the sink the first couple of times. I made a huge mess on my computer desk the first time I gave it a go.
You may not want ot do this if you are already getting a good head from the pour but I have a chocolate stout that I can't seem to get a decent head on no matter how I dump it into the glass. The syringe trick kicks up a nice foam and looks a little like the Guinness cascade while it does it.
I'm not sure where I read it first (on here maybe) but Guinness used to provide a syringe with their bottled stout.