two ideas come to mind at this point.
1. create a VIP that Tableau can reference and then the VIP has the ability to translate to the target destination. Virtual IP address - Wikipedia, the free encyclopedia
2. create a script (e.g. Python) that calls the Tableau REST API update datasource connection method in order to update the server address and/or port
Many thanks for those suggestions Jeff, I'll apply those plus talking to a certified Tableau partner here in the UK
and will let you know what transpired.