You'll have to convince the agent to short-check your bags, and it might be possible if you have a long layover, but if it's just about an hour (under 2hrs), you'll have a very hard time making a plausible case. In case you do have a long layover, you can try to claim that you need to access your belongings, say gifts for friends in SG, but it would be absolutely at the discretion of the check-in agent to allow this.
And even if they do and you successfully terminate your travel in SG, you may potentially be on the 'blacklist' for JAL for future flights. For one you'll definitely delay the departure of the flight to KL.
Best if you do not have any check-in bags.