I think you hit the nail on the head with your concern about compression.
I have a bag of synthetic scraps here that I am saving for a pillow project but I would never contemplate using high fill power down; I have how-ever thought about mixing the scraps with some old down salvaged from a 30YO sleeping bag and using the combination.
If I do make it I’ll be making it in the tubular bolster style to simply give some neck support and that mainly because i have a long skinny stuff sack surplus to requirements