Just a couple thoughts on this. I would avoid using a white leghorn since who knows what they mask. I know many (atleast in my experience) carry barring which would mess things up more than they are already. I say try to work with what you have for now. I would cross in an ancona with great mottling, any sign of it should be gone in a few breedings back to the mille fluer. The other advantage here is that anconas should have the same type as leghorns so if you find a great ancona with great type and mottling you have a good start on improving birds. Another way would be to try some of those mille fluer colored swedish flower hens. It isnt the best cross I know but some of them look so much like leghorns already and I have seen many with great mille fluer coloring, just a thought.