OK, have found a few breadcrumbs.
There is this documentation that says “By default, AI Workbench assigns ports starting at 10000 and 10001 for the local location’s proxy and service, respectively. This information is saved in the contexts.json file.”
I’m optimistic that “by default” means there is an option to override this value, but when I modify contexts.json and try to activate local I’m met with the disappointing message: “invalid local context. the proxy port and service port must be 10000 and 10001”