No there is no alternative IDoc for a characteristics in SAP. It does not really require a downtime, it only has to be well prepared that you can reduce the processing time to a minimum, I think no company who is running SAP in production can afford a downtime while new acquisitions get rolled in. You usually have a master data team in a project, a core team that is working most of the time in the project and an extended team that gets informed about the changes, so you usually have already contact to most of the people who maintain master data.
So an email introducing the changes can make them aware in advance and the can plan their activities and finally you set a message in SAP (SM02) when you start your update and when it is finished.