I don't think it does. I have all three types of Yahoo accounts - a freebie, a paid one with @yahoo.com address, and a custom domain with Yahoo being host. You can only use the Yahoo mail setup option on the iPhone with accounts that end in @yahoo.com. That part is hard coded into the email ID field. Those work with push fine.
The only way to configure the custom domain, even if it is hosted by Yahoo, is using the "other" option. The only configuration information I have for that is a POP address (pop.bizmail.yahoo.com). Perhaps Yahoo mail can be custom configured for IMAP access, but I don't know what that setting is. I tried defining an IMAP account with the Yahoo settings, even changing pop.bizmail to imap.bizmail, with no luck. I was guessing at that point. It seems that when you choose the Yahoo Mail configuration option on the iPhone, it is predefined for whatever Yahoo needs for IMAP, but you can only use @yahoo email accounts.