Double PAL / Double NTSC, oficially supported by WHDLoad...
 

Alrighty! DblPAL / DblNTSC is oficially supported by WHDLoad in the newest 16.5 version!

Uhmmm... So, like now what? :p

Is it true, that the slaves for EVERY game has to be rewritten to support this?
Maybe some "AmiDOS based" games, like donno... Dune 1&2/Monkey Island 1&2/Slam Tilt/Worms for fast examples, already can use it?

This, is like the most important thing, that keeps me from getting a better monitor (then my trusty, good, yet very old C=1084S)...

It will only work for installs which use kick31emu and depend on the games compatibilty. But if the game supports it a simple recompile should be sufficient.
Games which do any hardware access/tricks will usually not work.

How does this work?

Does this work by patching the copperlist or does it only work on OS friendly games that call OpenScreen or OpenScreenTags ?

It only works on OS friendly games that call OpenScreen or OpenScreenTags.
It needs/uses the original monitor driver files (must be copied into the games install directory).