"Thank you for reaching out to Efabless! To answer your questions, we can support requests for bare die in our QFN shuttle programs, with the next QFN shuttle having a tapeout of November 14, 2022. Also, the material that you were asking about is aluminum."
Correct me if I am wrong, this means I could do this. Perhaps I am wrong! In which case I should find a different chip supplier.
The padframe and top metal layer which "entomb" Caravel designs are both part of the die. They are not part of the package. Omitting the package does not help you here.
You might be confusing the padframe with the leadframe. The padframe is part of the die, the leadframe is part of the package. In a QFN package they are connected to each other by extremely thin gold wires.
"From the ChipIgnite program you can get bare die; from the Open MPW program, you can't. If you submit something on a ChipIgnite shuttle run, then the pads will be exposed, along with any other places on the chip that you place overglass cuts (i.e., you can place your own pad cuts internal to the user area)."
The other problem is that even for $10,000 the ChipIgnite program won't give you a confirmed booking. You have to submit your design and then they decide if they feel like manufacturing it. Any chip worth paying $10k to fab is going to involve an engineering sunk cost of around 5 times that amount. Which is why Real Fabs take your money and give you a confirmed booking.
Correct me if I am wrong, this means I could do this. Perhaps I am wrong! In which case I should find a different chip supplier.