Okay folks, it is a common myth that trainers don't work in multiplayer. Any person using the tools I can use can make a trainer that WORKS in multiplayer. Now, with trainers in multiplayer, if you do too much memory poking, it pretty much crashes the game. Also, it is most of the time easy to spot cheats being used, but with the right trainer subtle enough changes can be made so that the change will not be noticed by non-obcessive compulsives. For this reason, I am not going to publically say what you need to make a trainer. The tools I used are still publically available and can be found via the google search, and no real programming knowledge is required to make a trainer.
As for what you can do, Haxt0r,
Maybe you can do tripped out graphics.
You can freeze all of the units/structures for a particular player.
You can reduce the speeds of certain units for certain players.
Randomly placed mines, if at all possible, would require a heck of a good programmer and cannot be done unless it is a hidden cheat already in the game.
Changing the weapons, that is far beyond my skills if at all possible. You can create guard posts with randomly-changing-in-appearance starflare/supernova weapons.