Is that AA 6063-T52 alloy, 1 in × 3 in × 0.125 in rectangular extrusion? That’s weldable for sure, and decently machinable. If you weld it, the properties of the metal in the area of the weld will approximate AA 6063-O instead (which is the annealed, or softest condition for that alloy). Have a look here for more exact figures (-T52 is pretty close to -T5). Basically, the heat-affected zone and the weld material plastically deform (i.e. become permanently bent out of shape, as opposed to elastically deforming) much more easily than the tempered base metal. But the stiffness is essentially the same (modulus of elasticity doesn’t depend on heat treatment), so as long as you don’t exceed the yield stress in any given area, they’ll both perform equivalently.
If bumpers persist in 2010, there will probably be relatively few occasions where the welded joints would experience enough stress to yield them. So basically, I wouldn’t be too worried about it, unless you’re planning to expose parts of the frame. However, it’s a good idea to avoid placing welds where you know there’s going to be a serious stress concentration (because that’s asking for trouble).
One rule from 2009 that you need to consider is the requirement that bumpers be supported along their entire perimeter with something structural. (The precise definition of structural in this context wasn’t clear, but in practice, just about anything was acceptable.)
I’d personally use a bit more support for the cantilevered axles—one set of bearings near the wheel, and another set on the other side of the extrusion, as far away as possible. The size and position of those bearings depends somewhat on the game. If it was for Lunacy, I’d be comfortable with smaller bearings, because there’s really no chance of shock loading. But if there’s climbing (and therefore falling) involved, I might want to use a larger (stronger) bearing, or even a bushing instead, in order to make sure that there’s no bearing damage if the robot lands hard on its wheels.
One other issue with having the chain on that side of the wheels—if your bearings do get sloppy, a small misalignment or amount of radial play at the far bearing will be magnified at the location of the sprockets. You might end up with problems tensioning the chain, if you do it that way, because the axles will be free to wobble. Conversely, if you tension the chain, you might inadvertently misalign the inner races of the far bearings (by pulling them to one side). That reduces efficiency, and can cause bearing failure.