Excellent. This seems to solve the problem completely. I even tried mixing Arabic and English text in the RTL footnotes, switched on the “alternative=serried” option, and defined my own custom note streams — everything worked very well. The only thing that didn’t work is to mix, in the same stream, footnotes that are LTR with footnotes that are RTL. But I did not expect this to work, and I cannot think of a real world scenario in which such footnotes would/should be mixed in the same stream.

Attached are both the .tex file and the PDF with a perfectly working example. Many thanks to you both, Wolfgang and Hans. Hans, I assume we’ll know if the patch eventually makes it into a beta? 

Best,
Talal