Easily. Done it many times. If they are a flag hotel, they will have a sample login page in a few formats. Nomadix is the easiest to modify. You will then need to make it flat (All elements in one directory) and you will need to change the code to be SmallWall compatible. The "stock" captive portal is downloadable to see some clean code. Also, the needed posts and gets are on the actual captive portal page. The "file manager" tab is where you upload the .css and images, and embedded text pages.
Note that I would love some sample Captive Portal pages, so if you want to share, you will get attribution, and fame among the multitudes!
Yeah, this is an old thread but I have a need for a captive portal and this is my first go at it with Smallwall.
According to the m0n0wall docs the login page can be hosted on m0n0wall - or Smallwall in this case. Is that correct or do I need to redirect to another web server? I would really like to run a Radius server but I need to research that a bit more.