There is a bypass unit you can buy for the e9x & e6x. It is a little more complex than the e39/e46 version so it comes in at around €40

http://www.ebay.ie/sch/i.html?_trksid=p3907.m570.l1313&_nkw=E90+bypass+se nsor&_sacat=0&_from=R40

You can code them out too but you need to code out both the driver & passenger side, otherwise the airbag module gets bricked.

Of course the proper repair is to replace the occupancy sensor mat, that is the only recommendation people should really be giving on this subject.