If I read your question right, you are looking to find if Continuation will work in Visual Flow.
Based on the documentation, and all available resources, the answer seems to be No. By definition, Continuation is a way to make an asynchronous "long running" callouts only from Visualforce pages.
An asynchronous callout is a callout that is made from a Visualforce page for which the response is returned through a callback method. An asynchronous callout is also referred to as a continuation.
If you refer to the same link as above, it mentions that it is supported only through visualforce pages, excerpt below:
Asynchronous callouts are supported only through a Visualforce page. Making an asynchronous callout by invoking the action method outside a Visualforce page, such as in the Developer Console, isn’t supported.