You *might* be able to accomplish this with a bit of java script. YMMV, as I'm not completely sure this would work.
Here's what you would have to do. Use an external captive portal page on one of your web servers. On that external captive portal page, do an onload submit (if it's possible) to submit generic credentials back to securelogin.arubanetworks.com. That would then change the users role from a captive portal role to a guest role. The captive portal role would also have the redirect after login to the company website.