If the fare rules allow a stopover with no added cost, why not ask the TA to book it that way now? You can just note in your travel plans to exclude per diem on the extra day - almost always the case that you can tack on personal days to travel if no added cost to gov't (which there wouldn't be if you're paying for your own hotel and not claiming per diem).

The 14 hour travel rule is tricky because agencies may have specific policies when it's allowed. If you're returning home on a weekend the justification for a rest stop is pretty weak - you could rest on Sunday before returning to work on Monday.