I'm trying to make my own column and I've also hit this limitation.
It's already been discussed on this forum a few times (http://wpf.codeplex.com/Thread/List.aspx?SearchText=oninput).
In the original issue for this (http://wpf.codeplex.com/WorkItem/View.aspx?WorkItemId=8416), Ben Carter said "This is on our list of features to add in the future. The
TODO reflects the fact that the API had not been reviewed or tested as a public API but that it is recognized as an important API to have."
Does that mean it's safe for me to compile the source with PUBLIC_ONINPUT defined or should I look for a work around?