Even though Chromium is an open-source project, you have to remember it is administered, managed, and controlled by Google. And Google uses Chromium code to directly build and release Chrome browser.
Read between the lines. The simple reason there are no plans to expose sync server settings is because in that Chromium code that Google uses to build Chrome, Google does not want to expose such settings. They want all syncing to go through Google servers because they want your info.
Of course, any patch you submit for other sync servers has to be vetted by the Chromium dev admins, ie. Google staff. If they don't like what you're doing, probably like allowing other sync servers, they will disallow the patch. This is regardless of whether the patch works or not. Chromium devs also accept or reject patches based on whether they want such features in Chrome or not.
I have my suspicions that there would not be any home for a sync server patch in the Chromium community, and you would probably have to fork a copy to do your sync work.