Hello Michael -
this question frequently pops up on this list.
You can use layer 3 Batman to announce a default route, but this is not recommended. Default routes in a large mesh are a pain in the ****. Which is something we quickly realized in the Freifunk community as soon as we had more than one gateway or malfunctioning gateways - those will act as black holes and effectively interrupt your service.
You can circumvent the 0.0.0.0/0 check in Batman, by announcing two HNA's of 0.0.0.0/1 and 128.0.0.0/1 with the -a option. If you don't define a gateway class on the gateway the Batman gateway clients can't select a gateway.
Disadvantages: Other than the fact that I personally consider the announcement of 0.0.0.0/0 routes as sabotage - none that I know of, really. A few bytes overhead in the OGMs of the gateway, thats it.
We have occasional OLSR advertisement fade-ins on this list - I guess the Babel/OLSR/Batman quarrel will become proverbial one day ;-)
Cheers, Elektra