What it says on the tin. It seems that the prices of items are hardcoded to be in pixels, but I could be wrong. The terraforge is a crafting station, and hence isn't what I'm looking for.
Yeah there was a reddit post recently about this. Currently merchants are hard-coded to use pixels, so we can't yet. You could use a crafting recipe thing that takes the essence, but other than that, we're out of luck until the devs patch it in. There was a dev reply on the post, so at least they know.
He was the one that said it was hard coded. Also said that they need to go back and fix this, so it is a good bet that they will patch in support for custom currencies to be accepted.