On this front at least we (as developers) have all the same limitations that you (the modders) do. There are some things you can update withthout leaving the game (some table files, certain asset types), some that require going back to the main menu and reloading from there (most vehicles, characters and level assets), some that require a full restart (certain vehicle files - like rims or paints - weapons, animations), and a small number of things are updated live (effects, lights - although we're working on making far more things live update-able, it's too late for SR4). You likely won't get the live update stuff as that's dependent on things we can't make available, as Mike mentioned.
You guys actually have a slight advantage. In development the game is much, much slower to load than the version we ship, so making changes and reloading the game can take a very long time indeed. While it's frustrating, you can at least take solace in the fact for the majority of areas we didn't have a magic bullet that you guys lack.