Holfmann,
If there is dummy plant ,if people are changing the plant to maintain original plant then also route will be determine automatically for that original plant also based on below settings.
Departure zone-Shipping point
Shipping condition-from SH party
Transportation zone-from SH party
Transportation Group-MM03
While creating sales order system will check above data to get the Route.
Note:You cam make that route field in in-completion procedure as this mandatory.
Thanks,
Naren