I know this doesn't help in your quest, but... why don't you just use Excite Mobile?

Some problems w/ your scheme:

• You're going to find it difficult to get Excite cookies for yourself, unless you have an Excite domain. :) The browser simply won't send them to you.

• As far as I know Excite doesn't provide any simple methods of authenticating against their user database, from an outside service. You'd have to send a request like their login page does and then do something with the result, which would probably be either a redirect to the start page or a "failed" page.