There's actually a setting now that lets you control the minimum security level on crossposts. Go to Manage Accounts -> Other Sites, then click on the Details link for your crosspost account. There should be an option there for 'Minimum Security', which you can set to Public, Friends, or Private. If you choose Friends or Private, then that security level will be maintained through edits.

It's kind of hard to find, but it's there. :)