This is all moot anyway. No matter what you do with DNS if the client web browser is asking for an https connection and the captive portal gets in the middle, a certificate error must be displayed.
We, as IP networking professionals, should never, ever, EVER implement anything that, by design, will present certificate errors to users. Connections to https sites before captive portal is negotiated should simply hang. Don't like it? Don't use a captive portal.