FYI, you can use the Lua Console to make changes to your game without using Lua Events, so you don't
have to use Lua events just because you follow these instructions.
What I recommend is dividing your map up into "continents", and giving each continent its own landmass. You'll have 63 of them to play with. This will mean that caravan payouts are doubled between continents, but you already have various separate islands anyway, event if America and AfroEurasia share the same continent number.
Extract the contents LandmassChange.zip into the lua folder in your test of time directory. If you already have any of these files there, replacing them with the newer versions will be fine.
We'll now use a couple scripts to define and change polygons.
Open cheat mode, and press CTRL+SHIFT+F3 to open the lua console.
Press Load Script, and select PolygonScript
Select Tell about polygon script for some instructions (I've pasted some of them here.)
Press k to mark the corners of your polygon, then press backspace for a menu
Some other options can help you determine if you've chosen the correct vertices for your polygon. Then print the constructor to the console.
Highlight the polygon constructor and copy it. (everything after polygon =)
Paste the constructor into the changeLandmass.lua file, beside the entry for the landmass you want to set that polygon to. Note: sea tiles won't have their landmass changed, so your polygons don't have to fit the edge of land exactly. Remember to save the file after changing it.
Re-load your saved game (to get rid of the units that were used to mark your polygon corners). Open the lua console again, and load the script changeLandmass.lua
Check the landmasses to make sure the changes took place.
Let me know if you run into difficulties.
EDIT: If anyone else wants to use this, and needs it for a scenario with more than one map, let me know. I realised that it might not work as intended for multiple maps, but I won't bother fixing it unless there is a need for it.