I did some research (google) on the solari or split-flap departure/arrival or flight information boards. It seems there are still a few producers. Also the system has been adapted in various art-projects.
www.solari.it/eng/solutions/descrizione_ferrovie.php?catid=161
images.google.nl/imgres?imgurl=http
meso.net/disjecta
bandtcrowd.blogspot.com/2008/12/greatness-that-are-solari-and-split.html
www.gysin-vanetti.com/show.php?id=98
Although the technology might be complexer than it appears, i still like the option. One thought I had was to print the top with Arabic and the bottom with Latin signs, and vice versa. I just don't know if this system works in Arabic. I did see an image that show Arabic, though. I especially like the idea if these split-flap signs would be really huge (and would make a lot of noise).
Alternatively I was thinking of led or lcd display systems. And even manually changeable stencil letters/signs. But also here I have no idea wether we could make Arabic in mono spaced stencil letter forms.
Another idea was to use mirrors. In the Museum für Lichtkunst in Unna, Germany I have seen a very impressive text/light sculpture by Mischa Kuball using the mirrored disco balls. In our case, using only half a circle of attached mirrors would spread any line of text in different directions.
www.lichtkunst-unna.de/v2/index3.php
This brought me on another use of mirrors. Two mirrors in a 90 degree angle would split a projected text left and right and provide opposite scrolling directions. Using two mirrors, facing opposite directions, one for Arabic and one for Latin would project the two languages almost merged with different scrolling directions.
