I think you'll need two different copies of this field or possibly two different layouts for this. Use one for specifying this value for new records and the other for peforming finds. This is where using a different layout avoids having both copies present at the same time. You can put a new record button on your layout that switches layouts before creating the new record. It can even pop up this layout in a small floating window if you want.
Thanks! Having two layouts, one for browing and one for editing is a great solution.