Remember that part of the goal with Gluu was to use open standards: primarily OpenID Connect and SAML. If you externalize the login page, you are not using open standards. Thus, you are dooming your customer to a proprietary solution, where lock-in and high switching costs are the inevitable result. Yes... old Web Access Management products used to work like that. But also, customers would get locked in to old web access management products.
But with that said, if you don't care about standards, we are publishing a mechanism using the ROPW OAuth flow, which will return a cookie in the access token. A web client could take this cookie and write it as a domain cookie. We're about to publish a blog on this with a sample script. Possible--Yes! Advisable?