Yes, we have build a site that was meant to be used in arabic language (and sooner or later will be). Although it is possible, there are some inconsistencies in the backend.
1) you will have to write manual checks for switching reading direction. this will be dependend on the logged in user settings. this works good in many situations (tinymce), but somehow it doesn't work an all fields.
2) its not possible to switch directionality for every item in the admin UI. Serveral field descriptions are displayed at the top left corner of a text input field, although writing inside this textbox works RTL. you will have to make some tradeoffs according work amount <-> complete arabic UI

At frontend level everything (except metadata) can be achieved by css files, that can be comfortably loaded according to active language. Metadata stuff is a matter of template design, this works well if you know how to use translatable and i18n extensions.

In most cases it's sufficient you have textareas in RTL mode (that's what we did in the end).

Did you also set i18n? Don't know if this may have any effect, but I always used them together and SiteTree also (somehow) uses i18n.

Yes, it took me a while to figure this one out. Probably not the best solution, but it works. I've put it in the _config.php to make sure it's already set when SilverStripe outputs Backend stuff.