It is definitely difficult to drill down to the relevant info particularly as it relates to the coaching staff. The truth is I have no idea who is truly a great offensive line coach, and I imagine 99.99999996% of people in the world have no idea either. That's why I default to cohesion, if they keep the same guy year-over-year then the team at least thinks he's good enough. But, sure HC/OC may be a better metric (although good OCs become HCs so if they leave and the OL coach stays is that a knock on unit cohesion? I'm honestly not sure).
However I do believe that the number of consecutive starts a unit has together is a solid metric, regardless of talent level. Like the OL coach, or any staff member, the more times the team allows someone to do something the more confidence it suggests they have in that person (
Link). Unfortunately it looks like the only way to find out who started for which team on a week-over-week basis is to look at all 256 game logs. And if you want to go back more than one season, well...ain't nobody got time for that!
Best alternative I can see so far is to use Pro-Football-Reference to see how many starts/games played for each team. While it won't tell how many different starting combinations there were, you can use the 16 game starters as a base to work your way down and glean what you can from there. If I see a unit that had four or five guys who have started 16, 32, 48 (highly unlikely) games together and they are all coming back this year then I am giving them a bump in my OL rankings. YMMV.
If I decide to tackle this project, unlikely but...maybe, I will make a google doc to share.