I also have a "BIG" cavalier. He is 10 months and 18lbs. Very lean, just tall and long. I bought him from a respected breeder, and told him I was looking for a bigger pup for my 12 year old son. The breeder delivered! I'm hoping that he stops growing soon though, as he just fits in his bed and crate! I would talk to a breeder and see what you learn. Definitely would do that before tryign a cross-breed. Good luck!
I think there will eventually be a "Cavalier mix" breeding programme at some time, but it will be devised by geneticists and use carefully chosen dogs that are thoroughly checked for the health problems in both the parents' breeds.
Breeders that produce mixes are usually not concerned about anything but selling puppies for profit, although there is a UK doodle Club that appears to be doing more than most pedigree breed clubs http://www.ukdoodleclub.org/