Add syntax for "unchecked" relative URL

I'm currently assembling several docfx "mini-sites" together (for a project where the multi-versioning support wouldn't help). Occasionally, I need to cross-reference from one mini-site to another. I can do so via a regular href to the generated HTML, but I get a warning ("Invalid file link").

I definitely appreciate the warnings for other links (e.g. yml and md), but it would be nice to be able to tell docfx not to check a specific link, as I _know_ it won't be present at build time. (I have a later validation stage for that.)

Is there any existing syntax (ideally for both toc.yml and md files) that allows the target of a link to be unchecked?