In my experience, bare rooting coleus is a bad idea, especially the size plants generally sold at nurserys.

Given your zone especially, it probably makes more sense to pop the coleus into it's own pot of something closer to 5-1-1 without attempting to bare-root it.

I did (when first learning about the mixes) bare root some nursery-size coleus and plant it into gritty mix. Some died, some lived, but none were very happy with me. You'll also find the roots are tiny and get tangled immediately (if trying to really bare root it).

If you really want coleus in gritty mix, the best bet is probably to start with seeds. I did this too once, for fun, and it worked out pretty decently. But I find coleus thrives in 5-1-1, and isn't usually long lived enough to make gritty worthwhile.

Daniel