I haven't but would like to try it sometime. How I would do it is to start a historical game, with minors, for 1973, then:
1) Remove the major league affiliations for Bellingham (LA), Lewiston (OAK), Seattle (CIN), and Walla Walla (SD). You do this by going to League Settings > League & Teams, selecting (in this example) the Northwest League, and changing the affiliations to "none." This way all teams are independent and won't be affected by an MLB organizations up & down moves. (Not sure if it would be a concern to you or not, but this shouldn't have much affect on future major leaguers because very few players in the '73 NW League ever made the majors.)
2) Create a separate manager for each of the teams and have that GM/manager control (on the Manager Options panel, under Team Control Settings) Transactions, Initiate/React to Trades, Drafting Players, etc. You'll leave things like Setting Lineups & Depth Charts, Setting Pitching Staff, Player Strategies, and Active Roster Moves to Computer AI, promotions/demotions (from the reserve roster). The idea here is that anything that you want to keep the rosters static - no releases and free agent signings. So by assigning these to the Managers you created, none of these transactions will happen unless you go in - acting as or assigned as that manager - and make moves. However, by letting Computer AI handle lineups, pitching rotation & roles, player moves to and from the reserve roster, and so forth, you won't have to micro manage each team.
3) Go thru each team's roster, comparing baseballreference.com to OOTP, and move players as needed so your rosters are historically accurate. For the teams that were independent to begin with (Portland & Tri-City), you'll find that their rosters are already pretty accurate. For the other four teams that were affiliated, those players may be scattered about the organizations of their initial affiliation (that you removed in step 1).
4) You may or may not need to lock players to rosters in order to keep AI managers from releasing some of them. (You shouldn't have to based upon what was done in Step 2 above.)
There may be more steps, but this is probably a good starting point for replaying this league/season. And especially with OOTP there is more than one way to skin a cat. Others may have additional suggestions and/or other ways to accomplish this.
At any rate, once you've got everything setup and ready to start the season, I HIGHLY recommend making a backup, and then simply tesing everything out to make sure things are progressing in terms of rosters and player movement (and perhaps other stuff that interests you) in the manner that you expect. Then, if all looks good, restore from your backup and have a blast! If not, restore from your backup and start tweaking things...
Finally, the above is all under the assumption that historically-accurate rosters are important to you. If they are not then you can ignore most of what I wrote above
Good luck and report back!
