Never heard that solution.
As far as I know (which ain't that much) : the captive portal part isn't written so it permit you to put in a 'load balancer' option.
Faster, easier scalable is : more pfSEnse boxes and thus more separated "hotspot" zones.
With some 'correct' hardware pfSense can handle several thousand of logged in users - that has been seen before. Ones logged in, the load is close to nothing, the only issue will be 'how big is your WAN'.
Keep your login html simple.
Another issue : very recently (a couple of weeks ago), pfSense started to use a new web server : nginx. Captive portal settings for this server are pretty basic, and not much is know (yet) about optimizing etc.