You can do the same thing in a JavaScript Code Extension with the PreLoader scope. Just know that the "b" object and the "utag.data" object is not defined at that point so you'll need to use "utag_data." Tealium will grab the values from utag_data and will use them in load rules, extensions, etc.
"Instead of correcting the scenarios tab, there is no way that the excution time will be switched?"
Unfortunately no. As you can imagine this could wreak havoc with existing customers that are relying on the execution order they currently have. With that said, there are plans to re-do the scoping options. So instead of being limited to PreLoader, All Tags, and DOM Ready, you'll have options like, Before Load Rules, After Load rules, "As soon as Possible" :) Don't quote me on those but they will be similar. I do not have an ETA on that enhancement.