Hello,
Sometime ago I bougth a 2nd hand basketball set 3432, but without balls.
I tested it with balls from the "hungry hyppos" board game, (popular in Spain) , and fit well in the minifig hands. Have 14mm diameter.
I'm not sure, but may be an option as 3rd party ball or to test modules until the lego ball arrives
may be if some someone
in the forum that have both kind of balls , may compare them.
Anyway is a good contest idea.
Regards