I used to be able to do something like the OP wants to do traveling between DCA and LGW/LHR. I would buy the cheapest ticket I could get (there were four or five flights DCA-JFK), and then try to get on a departure in the 9 am hour which would get me to JFK around 11 am, and then I would use the 8 hour or so layover to attend the races at Aqueduct, which is quite close to JFK.
I'm not sure this is germane to what the OP wants to do, but a couple of years ago I was trying to get home from STL on the last departure of the evening, Unfortunately, thunderstorms were approaching STL, and the inbound flight which was to make up the flight to DTW was delayed and a certain misconnect was the result, and I couldn't get home until 4 pm the next day. It turned out that the SLC-STL-SLC turnaround was on the ground at STL and had not started boarding yet. I quickly called the PM line and got on that flight, which was the first leg of an STL-SLC-JFK-DCA trip that arrived at DCA by 9 am. I even got the last F seat on the STL-SLC (although I missed out on UGs on the other two legs).