osx Captive Portal with USB to Ethernet adapter
There's nothing special about "USB to Ethernet"-adapters regarding captive portals. Having a "special Apple adapter" will not help you in that regard.

You must find the error in your system somewhere else.

Actually I'm not even sure that Mac OS X looks for captive portals on wired networks. As far as I recall, the Captive Network Assistant is only activated for wireless networks. Your USB adapter would count as a wired network.