Technically speaking, the OP wants to do something that is not allowable within the contract rules for the fare being purchased. Specifically, fly airline XX from AAA-BBB-AAA and BBB-AAA-BBB to avoid some portion of the fare rules.

Can you get away with this once or twice, certainly. But to stay below the radar, others have suggested in the past to buy the tickets with some time and some good reason... like "I had to be back in city X for a meeting", or whatever. But the time component lessens the perceptions of fare rule avoidance (intent being a key factor in the equation). Evenralfp's pasting of NW's rules describe "FOR THE PURPOSE OF CIRCUMVENTING APPLICABLE TARIFF RULES" which is a type of intent.

On the other hand, as others have pointed out, you can fly a different airline. You aren't breaking airline XX's rules at all. Youare flying them on the appropriate days and are not buying other tickets from them to circumventing the rules. It's possible, but unlikely, that FF credits will give you away... but even then, there's no specific prohibition around flying someone else back to your home. As others have said, nothing prevents you from flying NYC-WAS-NYC and taking Amtrak, or driving, or whatever back to NYC in the middle. There's no "rule" that you have to stay within 200 miles of WAS during this trip. Just that you can't fly WAS-NYC-WAS in the middle on airline XX to avoid paying the difference in fares for a Saturday night stayover (for example).

I guess, my summary is this: 1) It's technically against the rules if it's your intent to have a lower fare. 2) You most likely won't get harassed if it's the one and only time. 3) If it is going to happen more than once, buy a 1 way or fly 2 different airlines... something to avoid the issue. And 4) If you HAVE to do this and you are doing it intentionally, do it with discretion... space out the 2 purchases if possible and DON'T CALL THE AIRLINE!!!