Historical/Fictional League(s) Idea

My idea is to run 8 separate historical/fictional leagues, each using a different era, at the same time. Then eventually take the top 2 teams from each league and play a season with those teams just to see how things turn out.

I chose 8 because it works out nicely, 1901, 1915, 1930, 1945, 1960, 1975, 1990, 2005.

I want to sim the entire baseball world for 15 years to build some history... so the 2005 league will begin w/ the 1990 settings, play through the 2005 settings, and so forth. The 1901 league will just have to sim through 15 seasons at the 1901 settings.

What I'm not positive on what to do yet is how to keep players from swapping leagues via trades or free agency. I would like to keep trades and free agency open within each league if possible. I think that if I create each league in a different country and disallow foreigners then that will solve the problem but I'm not 100%. Since I'm here at work tonight and can't set up some experiments right now I reckoned I'd ask for anyone's advice or opinions on how to set it up when I do.

Any other suggestions would be great.