Product Owner as a contracting career?

[Smartie]

Yes, I see your point - I'm just loathe to take the place of a permie as a PO. For the client's business benefit that knowledge should be kept in-house as much as possible in my view.

Agreed, it's not ideal. It is though a pragmatic approach to take when it will benefit the team/project. Ideally we need co-location and plenty of time from the PO but of course it's not an ideal world.
All the information on the product was shared with the team and I provided enough documentation on the process and system to make maintenance easy. There was also a (perm) Test Analyst who ended up with a deep understanding of the system.
So all in all, not exactly SCRUM but the project brought in a Minimum Viable Product on time, on budget and the Client was VERY happy :-)