Have done SYD-SIN-SYD
Very expensive
Daughter was on a tight schedule and was flying NRT-SIN-HEL
Her main ski boot developed a crack in the hard shell. She could have got a new boot but no time due to tight race schedule and the new boot would have required several visits to fit exactly.
She had an older pair at home and she only needed the shell, so I flew SYD-SIN, swopped out the boot shell and flew back SIN-SYD. All airside
I would also have done SYD-SIN-HEL if her flight was NRT-HEL direct. But not a round trip remaining airside
. It was serendipitous that the flights were booked via SIN
I've also done EK SYD-AKL-SYD all airside as well to bump up SC to get Tier status as I wanted to burn some FF points
I was not stopped /questioned but if they it would have been easy as I have a timeline developed in iPhone messages re purpose of trips