As of Anki version 2.1.28, the server address is not affected by the addon anymore.

What did I do?

I installed Anki 2.1.28 and created an addon with the following content:

import anki.sync, anki.hooks, aqt

addr = "http://127.0.0.1:27701/" # put your server address here
anki.sync.SYNC_BASE = "%s" + addr
def resetHostNum():
    aqt.mw.pm.profile['hostNum'] = None
anki.hooks.addHook("profileLoaded", resetHostNum)

Then I started Anki and hit the Sync button.

What did I expect to happen?

I expected Anki to make a request to the url specified in the addon.

What did happen instead?

Anki made a request to https://sync.ankiweb.net/msync/ instead.

Further information

The urls used for synchronizaton are hardcoded to https://sync.ankiweb.net since the commits 0e5b7da62aa0fd3726ca958f1cdae265d2dc0d91 and 529e89f48e747904ffd9c8ebac09739fb410bcaf.
Therefore they cannot be easily changed by the addon anymore.