From what I've come across, this is a lot easier to do in Panther than in previous builds. Note that this affects all aliases system-wide and you must be logged in as an Administrator.
Go to /System -> Library -> CoreServices, and change the owner for SystemIcons.bundle to yourself by control-clicking and choosing Get Info. Select Details under Ownership and Permissions. Be sure to change permissions back after completing this hint. If you're a terminal junkie, su and chmod your way to the next step.
Control-click on SystemIcons.bundle and select Show Package Contents. Go to Contents -> Resources and change the filename AliasBadgeIcon.icns to OLDAliasBadgeIcon.icns (or whatever your heart desires). Relaunch Finder and you should be set.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031206013833842