I'm not so sure those holes are "meant" to fit the accessory pins.
They are probably just close by accident, allowing you to put something in there. Last time I checked it was a pretty tight fit, which might cause damage to the parts.
If it was an intentional connection, we might likely have seen an official model/set make use of it by now.
As a side information, those tiny holes inside the bottom tubes of 1xX Plates/Bricks were not always there:
They began to show up somewhere around 1987-1991, mostly in newer moulds, modified plates like the 2x2 corner. Around 1996-1997 this started to show up in older regular plates too, since 1999 also in bricks. Oddly though some newer sets from 2000-2010 still occassionally had full pins for some reason (maybe even today?).
Parts before 1985 never had these holes.
They are probably just close by accident, allowing you to put something in there. Last time I checked it was a pretty tight fit, which might cause damage to the parts.
If it was an intentional connection, we might likely have seen an official model/set make use of it by now.
As a side information, those tiny holes inside the bottom tubes of 1xX Plates/Bricks were not always there:
They began to show up somewhere around 1987-1991, mostly in newer moulds, modified plates like the 2x2 corner. Around 1996-1997 this started to show up in older regular plates too, since 1999 also in bricks. Oddly though some newer sets from 2000-2010 still occassionally had full pins for some reason (maybe even today?).
Parts before 1985 never had these holes.