I think there should be clauses in the contract stating that if the player doesn't sign and mail out their cards by a given date, then the player will need to forfit a certain percentage of their autograph earnings. Maybe this will encourage players/agents to keep up with their signings instead of never returning them or returning them super late.
The NHLPA would no doubt have a problem with this, but I think putting something in place like this would be fair.
Good luck enforcing anything like that - you cut off one player, you risk cutting off all access to the agent's stable of players. At the end of the day, if a guy is making $8-10 Million/year, what does he care if he doesn't make an extra $50 K signing cards?