Using Domain Mapping plugin point a subdomain to a subdirectory

I'm on a subdomain multisite install. From what I can tell, with the domain mapping plugin it's not possible to map a subdirectory as the primary domain of a subsite. I.e. I can't use as the mapped domain for

This might be a question for @Sam but is this a technical limitation or is this just a feature that the domain mapping plugin doesn't support yet? If the latter, are there any plans to support it?

From what I recall the original plugin this one is based on supports subdirectory installs. Curious if it's just a superficial limitation or a technical one.