For our company, we utilize the comments field when files are checked back into Vault.
i.e. Added 1/4 fillet to outer edge.
Of course this is for our library part editors only. Regular day to day users cannot modify library parts at all.
I know what you are talking about. I have seen this feature in other softwares where you can look at the properties of a feature and tell who created it. Short of what has been suggested regarding the comments and possibly using an ilogic rule to tap into this seemingly hidden info, I do not know of a way.
You could write a bit of iLogic to create a "log" file...
You could track a lot more than just who created a feature that way, maybe track deleted featues, old dimensions, etc.