From a quick test it appears that the custom fonts work in KF8 (azw3) and KFX formats, but not the in the old MOBI format.
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I tried to test Arabic support, but so far I get a popup with "The item is not compatible with this device." when I try to select an Arabic language book.
I tried to test with the free sample for this book:
https://www.amazon.com/gp/product/B0774TM745
among others.