Use a field from one model, in another (REST datasource) model

Yes… the global merge variables are very handy. Not all the uses are documented. There are “secret” ones like model lookup and aggregate model merges and child relationship merges that I don’t think made it in to the documentation, but are floating around the community. For the most part, if there is information somewhere, you can merge it. You just have to figure out the correct format for the merge.