Maybe I'm confused, but I think you can manually enter data on autodata goals if you set the data source back to manual. On goals you create via the regular New Goal screen there's a radio button on the terrifyingly advanced settings:
Interesting! I was referring to automated-data-source goals created using Beeminder-side integrations, which IIRC had the other data source options grayed out when I looked (but it's been quite some time since I created one, so I may be misremembering that).
For goals you create via an integration, you have to email support.
Ah, okay, so maybe I'm remembering the grayed-out thing correctly, and a quick email would have sorted it for me. Good to know!
(N.B. the "Data source" choice doesn't ever prevent data from being added to a goal by an integration or the API, it just fiddles with UI elements)
FIRST SET OF THOUGHTS UPON READING THIS:
RIght, the reason Complice's new Beeminder integration feature does the awesomesauce I was describing is because it's from the Complice side, and effectively posing as me to "manually" enter data into Beeminder. And this does indeed work for anything you can integrate from the outside. You can authorize any number of apps to send data to your Beeminder account, from the apps themselves (assuming they offer that functionality or you can get there via IFTTT). What I was suggesting is enabling multiple Beeminder-side integrations.
So, for example, I set up my aforementioned guitar-practice goal as a Beeminder integration with RescueTime, and was then given one choice of RT activity to use as a data source for the Beeminder goal. (Again, this was a while ago, so that setup may have changed since I used it.) What I'm suggesting is that Beeminder let me first create a goal of the type I want (Do More, Do Less, etc.) and then associate it with possibly several input sources: let's say two RescueTime activities and an IFTTT recipe, for example.
But maybe this is really just a cherry-on-top kind of feature, rather than something that would actually make Beeminder more average-user-friendly (which was my thinking when I wrote my initial post). It's hard for me to gauge that well; it might still all be Klingon to the type of user I was thinking it would help.
SECOND SET OF THOUGHTS UPON READING THIS:
Wait, wait, I think you're saying that the Beeminder "create a new RescueTime goal" was just a convenience to get one automated data source linked to that goal, and I could have set up as many as I liked from the outside to go along with it. So I misunderstood the freedom I had. Good to know! And possibly worth documenting somewhere.