Hello guys,
Long time no see. Hope you will find this brief post useful.
To add custom filters to a N:1 relationship (lookup) in Microsoft Dynamics CRM (2011 and above), please read this post.
To filter a N:N relationship (many to many), please read this post (for CRM 2013) or this post (for CRM 2011).
This being said, as long as you build a valid fetchXML, you can control the LookUp window (both single and multiple) in a manner to meet customer’s requirements.
Enjoy filtering! 🙂