Who to access field of nested relationship in soql where first relation is simple and next is polymorphic

Question

I want to access custom field of Owner (User) fields where Owner is polymorphic relation it can be either User or Group. I tried following query:

SELECT Id, Email, Name, Lead.TYPEOF Owner WHEN User THEN custom__c FROM CampaignMember WHERE CampaignId='xxxxxx'

Relationship stack is: Lead > Owner (User,Group) > custom__c.

Answer 1

SELECT      field1__c, 
            field2__c 
FROM        Lead 
WHERE       Owner.type = 'User' 
  AND       CampaignID = 'xxxxxx'

EDIT:

Leaving my original answer in case it helps someone else.

I believe what you are looking for is this:

SELECT Id, Email, Name,
    TYPEOF Lead.Owner
        WHEN User THEN custom__c
    END
FROM CampaignMember
WHERE Field != 'value'