Clearpass may be the only way to have a diffrent webpage for a device type, but I think the easier solution would be to get a mobile-savvy web dev to re-create your on-controller custom captive portal to be more adaptive to the screen width.
Modern web devs are able to create pages with simple HTML5/CSS that adapt very well to diffrent screen widths, so the page items would become more vertical when showing up on the mobile devices.
I'm not arguing against Clearpass, but it seems like a big expense if this is the only problem you are attempting to solve with it!
Aaron