Hi Anton,
I have dealt with this in BO 3.1 and BI 4.0 and its the same behavior of not being able to associate the delegate search with "LovMaterial" object.
As far as i know, it is not possible to associate delegate search with this type of LOV object and I did not find any KBA or notes or ideas associated with this. I would suggest you to wait for any others to confirm or give any tricks to do this and if you don't get any you can go ahead with posting OSS note or idea accordingly.
Thanks
Mallik