MongoDB ReplicaSet reconfig when primary and majority don't exist
Yes, you can do this, and in fact with just 2 data centers you really have no other choice.   Essentially, whenever you do not have a primary you can still do a reconfig but you must pass in {force:true} to make it happen on a non-primary node.

This is listed in the docs for the rs.reconfig() command and in detail (with examples) as part of a tutorial which describes exactly what needs to be done in the scenario you described:

http://docs.mongodb.org/manual/tutorial/reconfigure-replica-set-with-unavailable-members/

If you are interested, I cover this and other availability issues in the (free) M202: Advanced Deployment and Operations course on MongoDB University