As Nate the great said, your best bet is PDF. A PDF formatted for the screen size should work well. Calibre can convert reflowable ebook formats into PDF for you, although I don't know if it will detect RTL in a MOBI.
Another option might be FBReader, although only recent versions of FBReader support RTL and I'm not sure the FBReader hack works on the KDX. See
FBreader & fbKindle.
EDIT: Calibre can't embed fonts, and you may need a PDF sized for the sceeen and with embedded fonts.