Adding a border to the top of a PDF form field

I'm working on a cerfiticate template for a client, and the number of signatures across the bottom can vary (not eletronic signatures--it is hand-signed). I have set up the name and title fields to show for print only if they contain informtaion, but the client has asked for a signature "line" above those fields when printing out the final certificate. Is there any way to add a top border only to a field when that field conatins text?