Wait, what?
There must be some drawback that you (and I) aren't seeing with this method. Because preventing someone from joining without a disconnect message was the whole point of dealing with the problems of using onConnectRequest. If there was a better alternative, someone would had to have found it by now
missionstartphase3ack is at least better than using autoadmincheck w/ disconnect because it lets them communicate on the server if they need help. They can then disconnect themselves after they download the client add-on. Then it will let them load in.
onConnectRequest would be preferred it that were feasible, but it turns out it will not work for a significant portion of blocklanders who have conflicting client mods. It would prevent a lot of the initial connect / disconnects when people are first getting the client mod. Oh well, at least there's the silver lining I mentioned above.
thanks everyone