I thought I'd share the results of an experiment I did.

Some time back I bought a Visitor Oyster Card so I could lend it to
people who were visiting me -- it has the advantage of being visually
distinct from a standard Oyster card.

Last week, I decided to try adding it to my online account, and it added
fine, and showed me the balance OK. It even *claims* that the card is
protected against theft, although I'm not sure what to make of that
since I'm sure that the Oyster T&Cs used to say that a visitor Oyster
can't be registered or protected.

But anyway, I successfully activated auto top-up on the visitor card and
I can also confirm that the auto top-up does work.

Just in case anyone's interested...

-roy