interesting question - don't know if it was tried already
here is an old thread for epub and the Sony reader
https://www.mobileread.com/forums/showthread.php?t=30512
with the thing I think you want to achieve
so - it's valid css to reference external fonts on your local file system
but I don't know how it is handled by the KF8 reader (will it be like a publisher font, can font changes still be made with the Aa menu etc)
perhaps you can ask in the Dev forum for a correct path to the user space on the Kindle and run some tests on your rig - please tell us the results
merry xmas