proof of crosswork:

without an online verifier (server or a blockchain), is there a solution for “i signed this no earlier than X in a way you can validate”?

any time someone finishes the MW with a crappy connection, it adds time.

would be cool to solve this but smells intractable

Or, if the device supported this (not sure if any do at the moment) you could use the device's secure enclave to sign the timestamp.  So long as you assume a user can't break the secure enclave (which is pretty reasonable in, say, iPhones) and you can attest to the version of code running, then this seems like a good solution, too.  But I am not at all familiar with how much devs can access secure enclave features in mobile devices.  But this is a good idea in theory.