Thanks for the advice.
I tried using the 3rd solution (div/span) but it canceled all the other formatting like list bullets etc. also it's a bit awkward to use a tag that closes after the whole text (if you need several paragraphs).
what I did finally is to write my own plugin called LTR that uses <LTR> ... </LTR> syntax and inserts a <div style='direction : ltr;'> in the document. in fact it's a slight modification of the 2nd example pluigin "color" (
http://wiki.splitbrain.org/plugin:tutorial#sample_plugin_2_-_color)
if you think it's worth publishing this plugin, let me know where.
Thanks again for the pointers.
Ariel.