As far as I can determine, any custom status is counted as Open. It would be great if this could be changed, but that is not the case. 

So if I had a ticket in status Closed, and then changed its status to a custom status I had made it would now count as reopned, in terms of the "Hours since reopned" condition? 

I am looking at a way for triggering "As we have closed you ticket can we now ...." follow ups and wondered if I could use a custom status to flag tickets in this state to reclose them after a while, or would this be better done using the "Hours since clossed" even though it is no longer closed.

It sounds as if you want to use the status code to trigger correspondence, while the status has not really changed.

One suggestion is to create a specific ticket field. With supervisor rules you can enter a value in this field x hours after ticket is closed (or other criteria).

You can also perform actions based on this ticket field, such as sending out an email to the requester. This ticket field should not be visible to the customer.

Depending on the number (and type) of follow-ups you want, you could decide on the type of field (e.g. drop-down list). For instance it might make sense to change this field to a specific value if a ticket is re-opened, and to skip this value for the "x hours after ticket is closed" condition.

Hope this helps.