Hi Alan
You can simply overtype the year if it is just an occasional old record. The JavaScript data picker control does support a yearRange configuration option (see http://docs.jquery.com/UI/Datepicker#option-yearRange) but we don't expose this via the data entry helper at the moment. This could of course be done though.
For vague dates, you'd need to use a simple text input rather than the date picker, or possibly use the date picker but disable client side validation otherwise jQuery will throw out any invalid dates. A better solution in the long term would be a custom date validation routine for jQuery which handles vague dates though I don't imagine that would be simple.
Regards