In my area, the last two rows were reserved for latecomers,
and in particular, those with small children. Sometimes it was reserved or cordoned off by
attendants.
Of course, most JWs find meetings dull as wall paper and would prefer to be at home or doing something else, or at least, not to have to go to the Kingdom Hall SO often. But they are under fear, obligation and guilt and feel compelled to attend.
There seems a general tendency that most JWs do not like sitting up front. Often, counsel was given from the platform to counter this, because I think many JWs would prefer to sit in back and not feel like they are being watched by everyone sitting behind them. If one has social anxiety, this sense is even more acute.
A DFed person who attends the Kingdom Hall has to endure allot of shame. It is an uphill battle for them to even walk in. I suspect this is so even for those without social anxiety.
Therefore, rather than a rule, it’s a natural consequence for a DFed person to slip in during the song, sit as far back as they can, be as invisible as possible (with exception of the elders to note their attendance), and slip out during the final song.