Yeah...if you have the ability to weld aluminum you definitely have the ability to make similar cookware. Whether it will be significant enough cost difference to make it worth your time is what I want to know. The DO's are like 200 or so, but by the time you buy material, cut it, weld it, and then have it sent off to be hard anodized (different then normal anodizing...has to be temperature controlled) it definitely adds up.
I have all the stuff to do it (and an intermediate level of skill), but the thing I've found is that fabrication companies usually charge the prices they do because that is what they need to float their business and not because they are greedy.
That said, I try not to take my time into consideration unless i have to take time off work...so its probably worth it in the long run. It may not look as nice as the Partner stuff, but I'm sure I could come up with something that works just as well and would be specifically catered to what I want.
As for texture, the texture from your typical cast iron items or even cast aluminum stuff is more due to the process then out of necessity or design. In my experience, smoother surfaces are easier to clean and stuff doesn't stick to them as easily and I can't think of a benefit of the rough texture that most DO's have. That rough texture comes from the casting process and is hard to get much better then you see it on most cast iron and aluminum.
Especially if we went the group buy route, I think any fabricator/welder on here could bang out a batch of partner style Rome Cookers pretty easily and make a decent profit still. The more you make the cheaper the material is per unit and the more worth your time it is since you can make multiple items from each setup.