Is it possible to override a field module's implementation of hook_field_validate()?
There is hook_field_attach_validate(), which is invoked by the single function that invokes hook_field_validate(), namely field_attach_validate(), which is called by field_attach_form_validate().

I did not find an obvious way to get rid of the original validation function, but a quick hack would be to remove the errors, that the original validation function produced, from the $errors array in your hook_field_attach_validate() implementation.