I’m trying to get my website, berkeleycitycollege.us/showcase, validated for its twitter card, but the tool does not seem to want to recognize that /showcase is not an extension of berkeleycitycollege.us. Is there a work around for this?
Twitter Cards are indeed generally enabled by type and domain name. What I would suggest you here is to submit approval for the berkeleycitycollege.us domain using your example URL, even if you eventually decide to place Twitter Cards markup only on the berkeleycitycollege.us/showcase pages.
Let us know if this is would work for you. Thanks!