From my point of view this concern is not really valid. Portal design may be nice to be more customizable, but it is nothing to do with security:
1. WEB authenticated WLANs are not secure at all, as they have no encryption.
2. Security of all Wi-Fi systems is standardized, so there is no much difference what exact brand you have, they all work exactly same way from security point of view. Exceptions are some vulnerable versions, but you must not use them anyway.
3. Anybody can identify wireless equipment used just by it BSSID (MAC address) -- you even don't need to be technically savvy, any WiFi detection utility shows equipment brand automatically. And no, you can't change MAC for Ruckus AP, if it is your next question.
Anyway, you can make your own splash page with any design you want, you just need to host it somewhere else, not on controller. We actually use Intel NUC boxes with dual-core CPU to host WISPR and to provide any customizations and additional functionality we need, it is just about 10x10x5 cm in size, but is a fully functional PC, not some limited Atom appliance.