I'm not sure that's actually necessary. You could just log a wal record saying "checksum this block" and if it gets replayed then recalculate the checksum on that block again. This record could be exempt from the usual rules for having a fpw.
There's no danger of torn pages from the checksum alone. The danger would be if some other operation does dirty that page then your need to know that the page is in this weird in between state where it's dirty but not yet had a fpw written.
I'm not sure whether it's worth the infrastructure to have such a state just for this or not. On the other hand it sounds like something that would be useful.