So you could implement a chip which reacts like an official passport. When the border guards see that the signature is invalid, you can explain how it's just a prank and you'll all have a jolly good laugh about it.
Both sides even have the info printed. One side in human format, the owner side in machine readable.
For a random traveller you can probably guess roughly how old they are, which is a few bits for the date-of-birth, and maybe you could strike up conversation and discover their name (or maybe it's printed on baggage, called out by fellow travellers etc.) but yeah it'll be very hard
For a very well known person you can likely discover everything except the passport number and you might get a decent guess at that from knowing roughly when it would be issued.
From a very well known person you could probably also steal everything you need directly, if your purpose is to create damage.
I don't think ICAO passports can ever be rewritten post-issuance. Some national IDs can, e.g. to change the holder's residential address, but for passports, I don't think any part of the on-chip data can be changed post-issuance, since it would also require re-printing data on the photo page usually under protective plastic.