Hi Brad, whilst many enterprise switches support an internal captive portal, many of them do not support an external captive portal in the same way as wireless controllers do. This is not really rocket science, its a pretty standard UAM implementation however many of them simply do not do it. Amigopod supports a completly custom external captive portal configuration where you can set the username/password syntax, post or get method, authentication method, etc. We have used this to interoperate with many NAS devices and if a particular switch does support UAM then we can integrate with it.
If you have a particular switch model that you are looking at or want to discuss fiurther feel free to drop me a line or reply to this post and we can take a look for you.
--Carlos
ps: have you looked at Arubas S3500 switches, they definitely have this support