Foot rot and shallow root system aside, sweet orange has a long history as rootstock and is said to produce great fruit. I want to play with it for container use. What would be the best variety to use? I will have Moro, Valencia and Sanguinelli available. Any input?

Valencia have quite a few seeds so that would be a good start.

It's probably a stupid idea, but all the old books and studies included sweet orange as a rootstock. I would really like to try it with an old line variety of orange on it. The combination is essentially the taste of citrus in the old days. I wish I could get Bidwell's Bar budwood from the CCPP. Perfect combination.

Sweet Orange is still in use in Australia, nowhere as much as other rootstocks. It's not used in replant situations. The seedlings grow well, but can be sensitive when repotting etc.
You may want to grow seed of all 3 vars, who knows which will perform better, there may be differences ? I would grow excess seed, and use the more vigorous seedlings.