Its a tough situation as in the 'ideal' world you would see signings through-out the off-season, but its hard to see a system that would make that happen. I do think they should have a signing deadline well in advance of spring training however. There are enough story lines in ST that I really don't want to still be wondering where the top FA's will sign during that time.
Something like an player can't play until the all-star break if not signed by date X or something. I think you would have to put in exceptions for guys that literally CAN'T find employment before then, due to being fringe or injured. Maybe you get around the signing date if your contract doesn't exceed $X dollars or something.