I get where you're coming from, but I think there's a selection effect that explains why this doesn't happen in practice.
Suppose you have a player with two or three years left on a deal that's at a major international and is likely to play. That player could sign a contract before the major international, but it's almost all downside for the player to do it. If they have a great tournament, they don't capture the value. If they sustain an injury, that's one step closer to a big-salary Bosman or a move to a bigger club at a cut-rate fee if they recover. The only risk is a career-ender or career-destroyer like Torres sustained (prior to the World Cup) - and even he got paid after the injury.
The guys that sign a contract before the tournament either are lesser players that won't play or have private information about known health risks. We can sign some lesser-rated players before the internationals, but we're going to have to wait on the bigger signings.