Back in 1999 or 2000 I remember a two day, two run, weekend race on South Mineral Creek and 1st Gorge Lime Creek. I think Dunbar Hardy had a lead roll in that event, but not sure. Regardless, credit is due, as it was a very progressive event at the time. It was a pretty sweet, albeit small event that drew a good spectator crowd to Adrenaline Falls (the top was the start of the Lime race). I remember a photographer was hanging over the lip. Both of those runs have decent access for spectators and boaters. There was event camping and registration at the campgrounds along South Mineral near town. I broke my boat on Friday doing training laps, patched it that night, and broke it Saturday morning on Lime above the starting line... so I didn't have a chance to post a time and always wanted to, but I don't think the event ever ran again.
OBJ truly is one of the best venues in all of Colorado. I think the regulations kept most of the OBJ races on a weekday though, which killed participation among us local weekend warriors. The weekday race was great for the traveling pros that were between events, but I would think a weekend race would have attracted more competitors and spectators, and thus sustained the event for longer.
The Upper Animas had a race that went through Rockwood at some point. I know it was the final leg of a Silverton area, Red Bull relay race.
As for future races...
Vallecito would be a burly race. It would be fun, but as noted, the flow window is narrow for it to be friendly. The safety logistics at Entrance Falls and a few other drops would need to be well organized.
High water Gore is certainly classic... but not easy to schedule with flows.
Baker's, Lake Creek, and Yule all have serious private property concerns, as noted.
On the safety side of things, I'm a believer that team races (groups of 2 or more) are superior over solo races with shore safety. The increased safety means that every competitor always have eyes on them, and prevents the need to set safety at every single hazardous location. This team format has made the best and hardest race I've ever participated in feasible... Rio Embudo!