Taking into consideration some more research and also points made in [this post][1], the end solution will be:

- (use code to) switch off the workflow enforcement on relevant schemas whilst a content port is in progress
- perform the import
- (use code to) switch the enforcement back on after the content port

We've had to introduce processes/checks to ensure that no-one sneaks in and adds/updates content whilst a content port is in progress and the enforcement is not being enacted - and we're still looking at the best combination here.

We did try, during the import, to intercept the various processes and prior to _Save > Initiate > Finsh_ but it seems that the check against enfrocement in the schema is undertaken and enacted on before anything we can hook into in Content Porter.
  [1]: https://tridion.stackexchange.com/questions/16182/updating-approval-status-of-a-item-on-save-event-system