Much of that instability when using a horizontal bar is basic physics so you will have the same problem, - to some degree.
There are two possible sources of instability...
1. Vibration from the cantilevered arm 2. Instability due to the tripod being off center (center of gravity shift).
Some weight hung from the back end of the bar will help with #2 but not #1. Have you tried that?
I have a copy stand and use wider angle lenses when I need a relatively large field of view. Or point the lens straight down from my Gitzos. I find it difficult to answer your question since it depends on subject size, camera/lens and expectations. I doubt there are any silver bullets here.
In general, though, the beefier the tripod the more you can push these envelopes. Depends how much money you want to throw at the problem. You can also add things like an RRS camera bar, which is very expensive, but probably somewhat more stable than a typical horizontal arm on your Velbon. Not cheap either, especially after adding all the parts to make it work (might need two heads, for example, plus a clamp).