Hi there. Have not tried that although I've tried something similar, Japanese egg custard, (chawan mushi) in the rice cooker. The result was a disaster: curdled custard.
I just can't remember if I used a bain marie when that happened. But also I suspect that rice cookers get too hot too fast for custards.
You may want to try it but make sure your ramekins are raised from the rice cooker bottom and are in a bain marie.
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