I fixed it, albeit perhaps suboptimally, with the following 1-minute hack:
Knowing a bit of networking, I needed to find the gateway's IP. One can either use ip, route, or even ifconfig for that. My IP was 10.5.50.174 and the gateway's, as provided by ip route command, 10.5.50.1. I put the gateway IP in the browser and was redirected to hotspot.plaza.local, which this time resulted in standard server-not-found error, indicative of a DNS failure. I didn't do much digging, but I figured hotspot.plaza.local and 10.5.50.1 were the same machine, but as is normally so, the server was configured to only answer on its virtual hostname, not the IP.
Afterwards, everything worked out as it was supposed to, except my relationship with Google Academy. Apparently, they don't necessarily greet an advantage-taking hacker in their ranks. :(
UpdateAs later thoroughly diagnosed, the problem was coorchestrated by:
- poorly constrived Microsoft's Active Directory with configuration, which set the DNS records for hotspot.plaza.local and plaza.local domains, but not for .local TLD; an amiss recommendation anyway,
- avahi-daemon (de facto mDNS/Zeroconf discovery service for GNU/Linux), which obeying /etc/nsswitch.conf therefore failed to lookup *.local domain with mDNS, and
- NoScript's ABE, which prevented loading of the hotspot's captive portal.
$ sudo service avahi-daemon stop
and, if running NoScript, temporarily disable ABE.