If you made a file like transform/ED001.map:
ON={out12:1}
OFF={out12:0}
and changed your binding config to:
Switch ED001 { mqtt=">[openhab:mytopic/test:command:*:MAP(ED001.map)]" }
I believe (haven’t tried it) that the ON command will be mapped to {out12:1} which will be used as the message to publish to mytopic/test.