First of all, I think your "in between" project has to be a graded long term work that is NOT a substitute for your main work, but a work in itself. So essentially the student is working on two pieces at once. You don't want students to "rush" through the first piece, just so that they can do the in between work. I have used "altered books" as a year long "in between" project. Altered books is where the student takes an existing book, does art on the front cover to establish a theme for the book, and extends the theme to the insides of the front and back cover. Then the student puts the book on the shelf in the room. (you would have a bookshelf full of your students' theme established books). Then during the year, other students would pick up each others's books and make double page spreads based on the theme established in the book. So essentially at the end of the year all books would be completed with works from each student, and the "owner of the book" (the student who established the theme) would have a book with their classmates' work and response to the theme. You, as the art teacher would grade it based upon your established rubric by asking each student to show you their completed page as they finished them.