RTL and LTR kerning in MS Word

Hello.

I am working on an Arabic Type that has RTL and LTR kerning.
- The LTR kerning (for Latin) has been imported once in the VOLT project using kern2volt.
- The RTL kerning (for Arabic) has been done directly in VOLT.

When I ship the font as ttf, both kerning works fine in inDesign and Word, both RTL and LTR
When I ship the font as otf, both kerning works fine in inDesign but only the RTL is working in Word (MS Word 2007-kerning activated).

I tried to remove the RTL kerning in VOLT and ship an otf with only LTR -> LTR kerning works again.
Having heard about problem with Word deactivating kerning when there are glyph without unicode involved, I tried to leave only one RTL kerning pair (reh-alef). This make the LTR kerning disappear again.

I came to the conclusion that Word won't accept kerning in two different directions with otf fonts.

(I could make a few test with Word 2010 and it looks as if the results were the same.)

Has anyone an idea or any informations about this issue?
Any help would be greatly appreciated.

Merci Beaucoup.
Sylvain.