Threads like these really make me wish we had gotten our free Madden codes by now...
On-topic, I think it would be a tough sell to the NFLPA, and the owners would probably have to give something up to get it. Additionally, a big part of why the current rookie pay system was put in place was to prevent contract hold outs (or more precisely huge contracts for unproven rookies to avoid those holdouts) - I could see players holding out to avoid this.