Originally Posted by jib71

Apparently it is possible but not straightforward to go from the air-side lounges through immigration and out into the arrivals area.

It's almost certainly easier to forgo the lounge, just come off your flight and enter Japan, and use the land-side showers in Terminal 1. Apparently showers there are free of charge for transiting passengers:
http://www.narita-airport.jp/en/news/omotenashi.html