any tax on profit should always be voluntary on both sides. The player should be able to negate it at all times, and so should the association be able to turn off the option. Because there are times when a player needs his money for more important things than a deposit for a worker 3 days later (new wagons, landmark, station, ...). And every player should be able to make his choice of when to / not to contribute to the corp. If the corp isn't satisfied with the contributions of a player, they can kick him out. No point in switching the responsibilities onto the players.
Any pool of money usable for a bid on workers, as you describe, should have a 20 to 30% fee (i.e. loss). Because worker bids are limited in time for a reason. If you don't want to spend 2min to log into the game and bid, you don't deserve to be able to bid as much as you want without a cost.
Otherwise, it's just much easier to remove worker bids altogether, and just give all workers to the best team. Because even tho they are usually the most active and win most of the workers, there are times where they only have 2-3 bidders (night) and can lose workers. With a new option to bid as much as you want whenever you want, as long as you deposit it before the auction, they would win every single one. Because the top team is also the one that makes the most profit.