What I did was go to the 70's player list and searched by team for my team the LA Dodgers. This gives you every player in the game that has at least 1 card for that team. I printed out that list and highlighted all the players that had the most ABs or IPs with LA.
The remaining players that had the most ABs or IPs with a team selected for this league I just scratched off the list. If there are players remaining that had the most IPs or ABs with teams NOT selected in this league and then your team then he is your property.
I probably didn't explain this very well but there it is. :)