Open arrangers are build around arranger software...

There currently are OMB, live-styler and Varranger.. Most people use mastrrkeyboards in combination with a standard PC, and other midi controllers... But obviously Varranger is kind of moving away from being open by restricting peoples freedom just to prevent them from making mistakes and running intoo stabillity issues.

Actually i dont think there is nothing available that would honor the name open arranger to the fullest