Hi Christine,
You can do it, but I see two potential problems:
1. Once you embed the ribbon, it is going to wick the moisture from the resin up the ribbon. You would somehow need that to stop that from happening. (I don’t have a good answer for that.)
2. You would somehow need to do the casting in two layers as well — and do it so the ribbon would lie flat across. By keeping it in the mold, the ribbon will come up along the side of the resin and more out the back. It wont come straight out the sides (in case that matters to you.)
I would anticipate casting the piece, popping it out of the mold, lying the ribbon across, then doming more resin on top. Here’s a video showing what I’m talking about:
https://youtu.be/DkZdO1n9R7Q