Can I assign a block to all pages than don't have one?

Lets say for simplicity I have 5 pages: page1, page2, page3, page4 and page5.

I have a header region, and have assigned specific blocks to go in there on pages 1 and 2 only.

Assuming the remaining pages aren’t particular to a role, content type, etc – is there a way using either the “Pages on which this PHP code returns TRUE” option to fill in the remaining pages 3, 4, 5 (i.e. if there’s nothing in this region, use this) or a module to do the trick that I can’t seem to find if so?

I’m basically trying to have particular header images where specified, but something to fill in the blank for any remaining arbitrary pages.

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