Don't think many operators are drawing circles these day as they simply do not reprsent a true picture of the range. As far as inserting ETOPS Entry and Exit point in the FMC, or for that matter the actual ETP Lat/Long I do no believe it would fall under best operating practices. Are you using a"tailored" Jepp plotting chart? May of the tailored charts have range arcs already printed.
Having said all of that I'm not familar with the 737NG FMC and what it can or cannot do. Can you put a Lat/Long on the FIX Page? Is it critical that you know the exact second you fall into ETOPS airspace? If you have marked the chart it seesm like you would be able to watch for the crossing??