The restriction isn't entirely arbitrary as far as I can see. If you allow the combination of inplace and openhook, then you will also need a writehook option, or the ability to specify a pair of hook function for the openhook. For example, there is a function provided for opening a compressed file for reading. In order to do an inplace edit, you would need to provide a write hook to compress the data on the way back in.

I think that the implementation of such a change would be open for discussion, and is beyond the scope of a minor change as suggested in this patch. (Perhaps the topic for a future PEP).