Although I don't know if there is a time limit, it is possible to exit the same station as you enter for no fee with the standard BART card. e.g. in case you realize you've forgotten something in your car after you pass the turnstiles. Therefore it should be possible to save 100% of fares for people leaving/arriving at opposite stations, respectively.

System-wide savings would then be proportional to directionality on a station by station basis - optimizing by zeroing out trips by longest first.

Dunno if this works with the clipper cards though, so everyone might need a magnetic card writer and blanks...