I did much the same as this back in July. I had booked CMB-SIN-ICN-SFO and return as one ticket and SIN-CMB and return as another.

Like you, I was concerned about picking-up the CMB originating ticket during the 1h25 turn-round, so booked a night in Colombo.

On the return (when I had the ticket in my hand for CMB-SIN) it was easy to follow the 'Transit' signs and collect the boarding pass without passing through immigration and customs.

The CMB station manager for SQ was very pleasant and efficient, and I suspect that you would not have problems collecting the CMB originating ticket at the transit desk. I seem to remember someone else posting earlier this year that they had done exactly this, and the station manager was waiting for him as he arrived with the ticket ready printed.

Best wishes for your trip.