I'm not sure exactly what the syntax should be, and I haven't seen this implemented in other search engines, but given duck duck go's support for user privacy I think it would make sense to add a list of public proxy servers as an instant answer. It would be cool if you could also add some filters. Like if you search for proxy list you would get all open proxies, but you could also search https proxy list, etc. For more specific results. I don't have any sources, unfortunately but I'll do some digging later.