You can access the Shape.len field, but you'll need to use bracket notation since the "." won't translate over in code.I tested this and it works with my SDE data.
var query:Query = new Query();
query.outFields = ["*"];
query.returnGeometry = false;
query.where = "AssetID <> ' '";
var queryTask:QueryTask = new QueryTask("http://testgis/ArcGIS/rest/services/SewerSystemTest/MapServer/5");
queryTask.execute(query, new mx.rpc.Responder(function(features:FeatureSet):void {
for each(var o:Object in features.attributes)
{
trace(o["Shape.len"]);
}
}, function(fault:FaultEvent):void{ trace("error", fault.message); }));