Whew, that took awhile. I don't want you all to get too excited, but here's a screenshot of the branch that I'm currently working on, scheduled to launch about a week after social launches next Monday/Tuesday (uggg, waiting on press so we can launch).
And just like view settings (original/feed/story), there's a global preference that you can override on a per-site basis, just as pictured above.
Also, river of news (reading by folder) is now instant, regardless of how many sites you have. (OK, it can take up to 1 second if you have > 50 sites you're reading from, but that's more about fetching the appropriate stories than it is figuring out which stories belong in the river.)
Just to confirm, not yet launched, but will very, very soon now. In fact, I've been testing it on a branch and jesus I can't believe I went this long without oldest-first. You guys are right as usual.