Oh, my current writeup has the ability for these weapons to be enchanted included, just that most folks with them don't bother doing so.
Ideas that I have so far:
1. Maximum enhancement bonus for Legendary gear is higher than for magic gear (+7 pre-epic). Crafting it requires ranks in the appropriate craft skill equal to or greater than 3 times the desired bonus.
2. Legendary gear may have more than one material in its construction, such as a cold iron/admantine alloy that takes traits of both.
3. Legendary equipment can only be improved (different from enchanting) by the smith who crafted it. This part is more to maintain the "legendary" quality of the smiths, as only they would know their work that well. It can be thrown out if desired with little mechanical ramifications.
All of the more "mundane" qualities (such as Keen, Whirling, Fortification, etc.) will be represented in improvement options. In addition, there will be some other benefits, such as cold iron granting a bonus on saves against magical effects (for the wearer, if armor).
Cutting through spells (effectively dispelling them) is quite possible for a higher-prerequisite improvement option (say 14 ranks in the appropriate Craft?)