The tilemap is just a FlxTilemapExt. Nothing fancy there.
I could grab tiles nearby entities. What I wonder is how the stamp would work. Not sure if there's a way to get the sprite of any given tilemap ? Plus, as you correctly pointed out, I'm afraid that the stamping would go outside the boundaries of said tiles. And yes, this is the purpose of the rectangles, to mask the stamps.
I might place all those rectangles manually, but sounds like a brute forced solution. I was wondering how other people might solve this, but sounds like isn't a easy task. Many thanks @jcox171 for your replies so far :)
Basically, what I did, was to extend 100 tiles the UpdateColumns() and UpdateRows() on FlxTilemap, also, changed the onScreen() to always return true. This could cause performance issues, but it is only used on small maps, for versus mode, so... is OK in this case.
In case it helps anyone, what I did was replacing my HaxeFlixel files, for @Nallebeorn ones: FlxBasePreloader.hx and FlxPreloader.hx, then on your project.XML file make sure you got: <app preloader="flixel.system.FlxPreloader" />
I use 4.0.1 version. If not mistaken, the next release will include this by default. I thought I might share in case someone else finds it useful.