It took me about half an hour of Googling to find this, and I couldn’t readily find this in the DWR documentation. Hopefully this may save someone some time:

To make a DWR call synchronous (meaning browser-blocking) you must pass an object as one of the arguments of your DWR call with a property async set to false.

I needed this feature in project where a collection object sends out a call for more data if it didn’t have it:

...
_getData: function(offset, limit){
    var r;
    DWRcomponent.getComments(offset, limit,{ 
        async: false,
        callback: function(s){
            r = toJSON('(' + s + ')');
        }
    });		
    return r;
}
...