BASEBALL?! The most boring sport EVAR?
Just kidding, glen. I'd be interested. Only keeper league that I've done is a hockey league on ESPN.com. I know in the past we've Yahoo for fantasy baseball and I believe you can do keepers there.
They're a lot of fun (granted, this is only our 2nd season in our hockey league), but I'm the league commish and I can dictate how many keepers you carry over from one year to the next. I think our league rosters hold 18 players with 3 IR spots and we are able to keep 6 players. After the keeper deadline passes, all players are released back into the market and can be drafted.
Again, this is a hockey league, so I'm sure baseball will be different but that's just my experience with keeper leagues. Luckily, we had all of our teams return for the 2nd season. That's probably the biggest thing about keeper leagues: continuity between players, and if one drops, will need to find another replacement.