It's possible. Most login pages would require https/SSL, so it needs a bit more than a basic browser. A decent homebrew web browser will be made eventually. For now, you can try the text only web browser Retawk in dslinux. Some hotspots only allow connections to servers on common ports like 80, 21 and maybe 6667. So even after logging in, you may not be able to use the non standard apps anyway. Some of the wi-fi apps use a lot of the DS resources and would be pushing it to add a background web browser but depending on the hotspot, you may only have to run the browser every 45 minutes or whatever their session time is set to.