All content gets packaged as add-ons now. I don't know why you would want to do it the other way.
1. Make an add-on that overwrites things, easy-peasy.
I asked because there isn't a user friendly way to manage client-side only add-ons. In other words, Blockland seems to just run every client side add-on it finds without the option to turn individual add-ons on or off. I only bring it up because of one idea I have involves a powerful feature based on #4, but it'd be potentially abusable since I have in mind a client command that's used to trigger it. I kind of get the feeling that the answer to #1 is 'no' because of the safeguards Badspot put in place. In this case, I might just have to go with protecting that client command with a prompt that requests permission from the player to run that command when the server asks. It'd be annoying, but at least a player wouldn't need to change servers that frequently. Better than someone abusing this on a non-trusted server and sending players to servers they didn't want to go to.
2. yes (stop it from sending data to the master server, then you just give out the IP to connect)
Yes, that's easier said than done, unless I missed that there's an option for it somewhere. I'm more wondering if it'd be as simple as overwriting a function with dummy function that does nothing, or if I'm going to have to rewrite most of the connection scripts to do this. If I have to rewrite a bunch of stuff, it could be difficult considering that most people don't have access to source of the main scripts that blockland uses.
3. Look in the advanced settings tab, you can change the host port (28000 by default) and I presume all ports after that are also used as connection ports (28001 - 28032)
Hmm, I'm going to have to look more into that. I would have figured the port number I used for the second server would have been close to the default, but it might just be randomly generated. Although, I was mostly hoping to see if this was possible on the command line. It'd be pretty annoying to have a separate blockland folder for each server just because I need to set an option for each one to specify the new default ports. On the other hand, I suppose it might be possible to just add a line in the bat file that changes the setting prior to running blockland. It'd work as long as that file only needed to be read in once per instance.
4. yes
I guess it would be more of an issue to find the correct function calls and what order to call them. Otherwise the only real reason I asked was to see if there was anything Badspot might have done to try to prevent this. A pretty good example of that is with the player jets. Making them more/less powerful should be a trivial matter, but Badspot did a good job of pretty much locking them to one setting.
