I imagine it's possible (I think certain AP's can be boosted by themselves to run at higher than the legally prescribed limits, for example the Linksys WAP54G / WRG54G can be made to do so with 3rd party firmware such as sveasoft).
However
afaik it's not just a case of 'it's not legal to sell equipment transmitting at above 18db', it's actually not legal to use it. Unless anyone knows otherwise?