For Arabic, we have to combine glyphs of the diacritics - that are called non-spacing combining marks - with the characters glyphs ourselves. Those marks take zero-space where they are placed above or under the letters they follow in the sequence, and their position in the characters sequence shouldn't affect the letters shaping and joining... So, I guess that arabjoin.c and bidi.c would be kept intact, and all the changes would be made in lcd-xxx.c
When I contacted HarfBuzz's developer - the rendering engine of pango - he agreed that it'd be difficult to use the code directly in rockbox.
it makes sense for the post to be under "user interface" indeed, I was confused about classifying it as I didn't find a topic that's directly related to "firmware"