Hi Andrew, thanks for bumping this topic!
We've just kicked off a big effort to enable community contribution to the Aggregate code and that work has to be finished first before any API work can begin.
You can read more about that effort at https://forum.getodk.org/t/odk-aggregate-future-plans. Your team's help there would allow us to move faster not just on Aggregate, but across the entire ecosystem.
As far as alternatives, I took a quick look at the Ona API and the Kobo API and both of those don't have the ability to add users programmatically, so it looks like the Selenium approach you have is likely the best alternative.