I'm not Robert but I am going to jump in with some thoughts. Under the right circumstances that might work but there are a lot of reasons you may have trouble with it. Robert can jump in when he catches up.
First, If your panels are under stress when you start the welding, you will never get it welded without it moving on you. If they are not under stress alligning a butt weld is no big deal.
Secondly, it is much more labor intensive to flange a joint, cut it off, leave the tabs, and weld it than it is to attach your panel with a few drill screws, scribe your cut line, cut it and butt weld it.
Thirdly. Your standard butt weld can easily be finished on both the front and back side making it a much nicer looking job.
If you feel you must. A small piece of scrap with a couple drill screws can hold the panels until you get it tacked. Then remove the drill screws and weld the little holes left and you are done without the tabs remaining. Personally, I usually use magnets to hold my panel until it is tacked. If the magnets won't hold it, I am probable going to be concerned about the fit anyway.. Just don't weld right beside your magnet.
The bottom line is, there is no purpose for reinventing the wheel. Any way you do it is going to be very difficult if you don't have solid, clean metal, fitted properly without being under stress, and the basic welding skill which you need anyway.
One other thing you need is lots of patience. plan your job well and enjoy the challenge. Metal working is addictive. The more you do, the better you get. The better you get, the more fun it is. The more fun it is, the more you do!
You get the point. Enjoy.
))
John