Great question — answering as the UphillTech team:
Steel cut oats take a lot longer to cook than rolled or quick oats. It's unlikely they will completely cook before the cookie is done. You could try it, but I'd recommend presoaking them in hot water for an hour, or maybe even parboiling them for a few minutes. You'd need to adjust your wet ingredients accordingly as well.
Full answer by hobodave: https://cooking.stackexchange.com/a/5326 (CC BY-SA 2.5).
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