The key is really lighting. You can "fake" a seamless if you light a big thing (preferably a light thing) in a way that will render it plain and bright white. Not that white b/g= Avedon, like some people tend to think automatically whenever they hear "white b/g", but from a technical standpoint, he is worth mentioning. For "In the American West", Avedon had the advantage of the largest and farthest away light source on Earth, taking his portraits/typologies outside in diffuse light. The distance of the sun and clouds made it so that the differences between subject and backdrop were not huge in a relative sense. You don't have that with basic studio lighting. The falloff due to the inverse square law is far greater in the studio, due to the relative distances between lights, subject, and b/g. This means you need dedicated b/g lights, in other words, and ones that are stronger than your main light. If you just have a main light for the subject, and maybe one for a fill, you will have a tough time even on a white seamless.
Also, don't forget that Avedon used very little of what he shot for that series, and trashed the rest. So, practice and repetition can be key as well.
