I dont know if this is the correct forum to post this in, but is there a simple way to alter the rules of a unit, say a warrior, so that its nationality is hidden? Not like spies, where they are invisible, but like privateers, which can be attacked, but you can't tell what nationality they are.
A unit like this could be useful in crippling a civ's economy without declaring war. This would be a powerful advantage, so it could be a national unit, that you can only have a limited amount of.
Any info would be much appreciated.:)

I know this is old, but I thought I'd give a quick answer anyhow.

I don't have time to do a full rundown on how to edit XML files or the like, but fortunately there are numerous tutorials and helpers on CivFanatics. I recommend starting here: http://forums.civfanatics.com/forumdisplay.php?f=158

I can tell you that it is very possible to do what you want, and easy too.

The behavior you are talking about is controlled by the following tag:

<bHiddenNationality>0</bHiddenNationality>

This is found in the CIV4UnitInfos.xml file. You would simply want to clone a unit (again I refer you to the various guides on these forums) or, more simply, you could just give one of the existing units the ability.

To activate the ability, just set the 0 in the line above to 1. This will hide the nationality of any unit you choose.

Land Privateers are realized in

Dawn of a new era mod. Details see here: http://forums.civfanatics.com/showthread.php?t=316383
Warning this mod is not compatible with the Civ4 Colonization patchs!!!

The same Land Privateers you can find also here: http://www.civfanatics.ru/forum/index.php?showtopic=9065
Link for mod here: http://www.rapidshare.ru/1018343
This mod called as "Model Europe" is in English and required Colonization + 1.01 patch.