It sounds like you should have multiple Content Types which inherit from one common Content Type rather than cascading dropdowns. I can see the complications that you're describing, but I'm guessing it's more of an information architecture issue thatn anything else.
The bottom line of your question, though, is that SPCascadeDropdowns won't work with the Content Type column. You could, however, create a 'proxy" column which contains the Content Type, and is a lookup into a list containing those Content Types.
M.