I find BeOS mainly curious because it experimented with strange features on each level (e.g. said tabs, BeFS/Tagging, etc). I think every one of those features should (and probably would) have gotten an overhaul by now if Be Inc. was still around.

That said, I haven't looked much at their APIs. If the tabbing behaviour is exposed properly, I can imagine it would be quite easy to write a small utility that stays in the tray and does that automatic tabbing separately.