@meem: None. But if you happen to know a good & complete Latin+Arabic/Hebrew free font family, I'll happily convert/package it
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@Balerion: Yeah, I may not have been very clear in my answer ^^. What I meant is that it's doable in this very specific case (override Sans_BoldItalic), because it's used almost nowhere else except for displaying the Collections, so you're not 'messing up' part of the styling in your books or the Kindle UI.
So, you can safely override *only* Sans_BoldItalic, but you can override it with *whatever* font you like, not necessarily Sans_Bold or Sans_Regular
(for example, if wanted to make the books title & authors in the Home screens non bold, that wouldn't be so nice, you'd have to sacrifice the Bold styling in your books, because the font used is Serif_Bold, but it's the font also used to display bold styling in your books/dictionaries).