Any risks in creating custom OS X extended attributes?
So far the only risk I've found is that some applications will overwrite the custom extended attributes.  TextEdit is one such example.  It will remove the attribute and add a quarantined attribute in its place, which will cause the Finder to give you a warning (that it was downloaded from the internet) when you open it.

So if you don't want to lose the metadata you've added to your custom attributes, don't open in unsafe applications like TextEdit.  TextMate and SublimeText 2 and vim don't damage your data, however.

(And as for name collisions, just follow the reverse DNS naming that Java uses.  For example 'com.yourdomain.yourcustomname'.)