I have so few whites in the garden. Whites are always on my want list, but it's difficult to find a hardy plant that performs well and has a near white bloom in the north. Good luck with your hybridizing for whites David.
The best white I have is an oldie, Whiter Shade. Branching could be better, top and some 3 way.
See Me-Feel Me-Touch Me is registered as ivory and it does have a yellow cast to it. If your looking to work with teeth + white SMFMTM or maybe Buzz Saw might be worth a look.
Have you seen Culvers Alpine Junction, Tundra Bird or Bristol Cream. I keep meaning to order Bristol Cream....Alpine Junction looks good too....
Michael Miller has some very nice white intros. I have an order coming from him this spring and may add one to it, if I can make up my mind which one...