This is the best I can do. If 11.4 doesn't provide a new enough fribidi there is nothing I can do... It's not an optional feature. We need to test the latest libass in Factory even if it is not available in 11.4.
I suggest a "osc copypac -r multimedia:libs libass multimedia:libs libass-pre121" or linking the latest fribidi in multimedia:libs.

- Update to version 0.10.0
+ Bidirectional layout and Arabic shaping via FriBidi (GC #13)
+ Add API for shaper configuration
+ Add support for `Language' Script Info property, this can be
used for hinting the text language
+ Vertical layout improvements
* Use `vert' and `vkna' OpenType features for vertical glyph
variants
* Position rotated glyphs onto baseline
+ Parse font encoding property for base text direction hinting
+ Refactor cache system
+ Use generic outlines in place of FreeType glyphs
+ Direct outline bitmap rendering
+ Fix whitespace trimming (GC #35)
+ Do not render border if there's no shadow or glyph (GC #29)
+ Adjust spacing after a italic to non-italic style change (GC #37)
+ Fix fade timing
+ Fix x positioning with borders (GC #27)
- Use pkgconfig() BuildRequires
- Parameterize the soname number