Perhaps an obvious question but... since you modified the legacy template are you sure you configured vB to use the legacy template?
A similar request was posted here, sounds like you were right the first time (field5) :
http://www.vbulletin.com/forum/showthread.php?t=96719
I'm looking for the same thing, only I want to know how to use conditionals so that if the user leaves the custom name blank it'll default to the registered name. I haven't found any answers on that part yet