You are right.  You cannot add a custom field to the Pages object because it is managed. 

However you can use the Module field to capture status information.  You can add picklist values to the field and then use these values to indicate where the page is… 

We tried that approach, but the module value contributes to the external id “skuid__UniqueId__c”.

If we export the pages and import into another sandbox, the pages with new module value will be created as new pages, and old pages with old module value will be redundant.

Have you tried exporting page versions instead of pages.  You could use the Description field and only export versions marked as “Production” using your automated process.  The Version record looks up to the Page object where you could ensure that unique Id field stayed the same. 

Its a wild guess and probably is not as solid as you want.   I don’t think we have a better solution for you right now. 

Am I wrong in assuming that you could create your own clone of skuid_Page__c? Could you create your own custom object with a reference field to the skuid Page, and add that field to the table?

I don’t think you could get Skuid to work against a different object for the Pages.  

I was thinking about the lookup route though.  A field that looked up to a particular Page record might be possible.  May be worth a shot.